Hydroxypyridonate and hydroxypyrimidinone chelating agents

ABSTRACT

The present invention provides hydroxypyridinone and hydroxypyrimidone chelating agents. Also provides are Gd(III) complexes of these agents, which are useful as contrast enhancing agents for magnetic resonance imaging. The invention also provides methods of preparing the compounds of the invention, as well as methods of using the compounds in magnetic resonance imaging applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 10/194,502, filed Jul. 12, 2002 and issued as U.S.Pat. No. 6,846,915; and claims the benefit under 35 USC 119(e) to U.S.Provisional Patent Application No. 60/312,132, filed Aug. 13, 2001, andto International Patent Application No. PCT/US2002/026026, filed Aug.13, 2002, all of which are incorporated by reference in their entiretyfor all purposes.

BACKGROUND OF THE INVENTION

It was first shown in 1972 that by superimposing linear field gradientsto the static magnetic field of a nuclear magnetic resonance (NMR)experiment it is possible to obtain three-dimensional images of anobject (Lauterbur, P. C., Nature 1973, 190). The new technique becameknown as magnetic resonance imaging (MRI). MRI has developedimpressively, becoming one of the most powerful tools “to look insidematter” (Aime, S. B. et al., E. Acc. Chem. Res. 1999, 32, 941). As X-rayimaging did in the beginning of the 20th century, magnetic resonanceimaging (MRI) has revolutionized modern diagnostic medicine (Caravan, P.E. et al., Chem. Rev. 1999, 99, 2293). Whereas conventional X-rays showskeletal structure, MRI enables the acquisition of high resolution,three-dimensional images of the distribution of water in vivo. Thispowerful diagnostic tool is invaluable in the detection of a widevariety of physiological abnormalities including tumors, lesions, andthromboses. Additionally, recent advances in dynamic MRI open up theexciting possibility of real-time imaging of biochemical activity (“ANew Generation of In Vivo Diagnostics,” MetaProbe, 2000).

The medical utility of MRI is enhanced through the administration ofcontrast agents prior to the scan, which alters the relaxation times ofprotons in the vicinity of the agent, increasing the degree of contrastbetween healthy and diseased tissue. The use of contrast agents isincreasingly popular in medical protocols, with some 30-35% of MRI scansnow acquired with the aid of a contrast agent (Caravan, P. E. et al.,Chem. Rev. 1999, 99, 2293; Aime, S. B. et al., E. Acc. Chem. Res. 1999,32, 941). Consequently, contrast agents now represent a very largemarket, with sales of over $250 million in 2000 (“RC-211, ContrastAgents for Medical Imaging,” Business Communications Company, Inc.,1999).

Several new contrast agents are currently under development, which aredesigned to be more site-specific, facilitating, for example, detailedimages of cardiovascular features (Lauffer, R. B., Magn. Reson. Med.1991, 22, 339). Additionally, recent reports have demonstrated thatcontrast agents can detect the presence of enzymes and metal cations(Moats, R. A. F. et al., Angew Chem., Int. Ed. Engl 1997, 36, 726; Li,W. F. et al., J. Am. Chem. Soc. 1999, 121, 1413).

At present, clinically accepted contrast agents are based upon agadolinium complex of a poly(aminocarboxylate) ligand, e.g., thegadolinium chelates of DTPA, DOTA, DO3A and DTPA-BMA (FIG. 1). Theagents are extracellular agents that distribute non-specificallythroughout the plasma and interstitial space of the body. A typical useof such agents is in the detection of tumors in the brain.

The image enhancing capability of available agents is far lower than theoptimal values predicted by theory (Aime, S. B. et al., Coord. Chem.Rev., 321: 185-6 (1999)). The relatively low image enhancing propertiesof current contrast agents requires injection of gram quantities inorder to obtain satisfactory contrast in the resulting image. With suchlarge doses required for reasonable image enhancement, present contrastagents are limited to targeting sites where they can be expected toaccumulate in high concentrations. To accomplish greater resolution withlower dose and to enable a variety of target-selective imaging (such ashepatobiliary features), there is a need for contrast agents ofincreased image enhancement capacity and corresponding enhanced waterproton relaxivity. Moreover, a useful complex must be highly watersoluble, resistant to in vivo dissociation of the metal ion from thechelate and of acceptably low toxicity. A promising new classmetal-binding ligands are based upon the heterocyclic pyridinone andpyrimidinone nuclei.

Distinct in both structure and properties from poly(aminocarboxylate)chelate-based MRI contrast agents is a class of compounds that includeone or more hydroxypyridinone or hydroxypyrimidinone subunit. Bothhomopodal and heteropodal chelating agents incorporating ahydroxypyridinone or hydroxypyrimidinone moiety are known in the art.Although many of the reported compounds exhibit the desirable waterexchange kinetics and complex stability characteristic of this class ofcompounds, the reported water solubilities of the Gd(III) complexes aregenerally insufficient to allow the complexes to be considered ascandidate MRI contrast enhancing agents.

For example, Xu et al. (J. Am. Chem. Soc., 117: 7245-7246 (1995)reported the synthesis and characterization of Gd(III) TREN-Me-3,2-HOPO(tris((3-hydroxy-1-methyl-2-oxo-1,2-didehydropyridine-4-carboxamido)ethyl)amine)).The solubility of the disclosed complex in water is only about 0.1 mM,making it less than ideal as a MRI contrast enhancing pharmaceutical.

Furthermore, Cohen et al. (Inorg. Chem., 39: 5747-5746 (2000)) prepareda series of mixed ligand systems that are based on the TREN-Me-3,2-HOPOplatform. The ligands include two HOPO chelators and a non-HOPOchelator. The ligands set forth in Cohen et al. incorporatesalicylamide, 2-hydroxyisophthalamide, 2,3-dihydroxyterephthalamide andbis(acetate) as the non-HOPO chelators. The Gd(III) complexes of theligands according to the disclosed motif were of moderate watersolubility (approx. 1-3 mM).

Hajela et al. (J. Am. Chem. Soc. 122: 11228-11229 (2000)) prepared ahomopodal Me-3,2-HOPO chelate based on a functionalized TREN backbone.The functionalized TREN backbone was a homochiraltris(2-hydroxymethyl)-TREN-Me-3,2-HOPO. The Gd(III) complex of theligand has a water solubility of approximately 15 mM.

HOPO ligands in which the endocyclic nitrogen of the pyridinone moietyis functionalized are known. For example, the ligand TREN-MOE-3,2-HOPO(tris(3-hydroxy-1-methoxyethyl)-2-oxo-1,2-didehydropyridine-4-carboxamido)ethyl)amine)and its Gd(III) complex was prepared and characterized by Johnson et al.(Inorg. Chem. 39: 2652-2660 (2000)). The complex was reported to have awater solubility of about 1 mM.

In addition to those references discussed above, U.S. Pat. No. 5,049,280discloses homopodal chelating agents based on the2,3-dihydroxyterephthalamide moiety. The '280 patent does not discloseor suggest combining the disclosed moiety with a HOPO or HOPY subunit toform a heteropodal chelating agent. U.S. Pat. No. 4,698,431 discloseschelating agents having 1-hydroxy-2-pyridinone subunits. The complexesare disclosed to exhibit a high affinity for iron ions and actinidessuch as Pu(IV). Neither the '280 nor the '431 patent suggest the use ofthe novel chelates to complex Gd(III).

U.S. Pat. No. 5,892,029 and 5,624,901 set forth a class of homo- andheteropodal chelate systems having at least one 3,2-HOPO subunit withintheir structure. Neither the '029 nor the '901 patent suggest that thedisclosed ligands are of use in forming a highly water soluble Gd(III)chelate.

Other related art includes U.S. Pat. No. 4,666,927, which discloses anumber of chelating agents having 1,2-HOPO, 3,2-HOPO, or 3,4-HOPOmoieties incorporated within their structures that are linked through anumber of possible combinations of linking groups, including —CONH—groups. However, U.S. Pat. No. 4,666,927 teaches against a HOPO moietyhaving a substitution ortho to the hydroxy or oxo group of the HOPOring. U.S. Pat. No. 6,221,476 discloses polyhydroxypridinone ligandsthat are attached to a membrane support. The compositions are useful forremoving metal ions from solutions. Zbinden (U.S. Pat. No. 5,688,815)sets forth a class of 3-hydroxypyridin-4-ones, which are effectivechelators of iron and useful to treat iron overload. Neither the '927nor the '476 teach one of skill in the art how to prepare a highly watersoluble Gd(III) chelate that is of use as a contrast enhancingpharmaceutical.

As discussed above, Gd(III) chelates of hydroxypyrimidinone andhydroxypyridinone ligands have a number of properties that make themsuperior MRI contrast agents relative to the widely usedGd(III)-poly(aminocarboxylate) agents. The development of the newcontrast media based on the pyrimidinone and pyridinone ring systemshas, however, been hampered by the inadequate water solubility of suchagents. A new generation of Gd(III) complexes based upon theheterocyclic ring systems would represent a significant advance in thefield of MRI contrast enhancement. Quite surprisingly, the presentinvention provides such complexes.

BRIEF SUMMARY OF THE INVENTION

Metal chelates and chelating agents are pharmaceutical agents that areuseful in both diagnostic and therapeutic applications. The presentinvention is directed to a new class of highly water-solubleparamagnetic metal chelates of use as contrast agents in medical imagingmodalities, such as magnetic resonance imaging (MRI). The paramagneticchelates of the invention have unexpectedly high water exchange rates,and correspondingly high proton relaxation rates, making them highlyeffective MRI contrast agents. Moreover, the synthetic pathways to thechelates of the invention provide for the facile incorporation ofsubunits that modify one or more properties of the chelates. Thus, thereare provided chelates that include water-soluble groups, targetinggroups, chelates conjugated to diverse macromolecules and the like.

The present invention provides a new class of highly water solubleGd(III) complexes that also exhibit excellent stability and resistanceto dissociation in vivo. Moreover, the Gd(III) complexes havesurprisingly rapid water exchange kinetics, giving the compounds potentrelaxivity. Thus, the invention provides complexes that are watersoluble, powerful, and non-toxic proton relaxation agents.

The present invention provides thermodynamically stable Gd(III)complexes of HOPO ligands that have relaxivities approximately twicethose of commercial agents such as Gd[DOTA] and Gd[DTPA]. The stabilityof the Gd complexes is also reflected in the preference of the ligandsfor Gd(III) in the presence of other metal ions. Moreover, selectedcompounds of the invention have relaxivities that are at least abouttwice those of commercial agents. In addition complexes provided by thepresent invention have near optimal water exchange rates, which areabout 100 times greater than existing agents.

Thus, in a first aspect, the present invention provides a complexbetween a Gd(III) ion and an organic ligand comprising only oxygen donoratoms coordinating the Gd(III) ion. The complexed gadolinium ion has awater exchange rate of at least about 10×10⁶ sec⁻¹. The solubility inwater of the complex is at least about 15 mM, preferably, at least about20 mM.

In a second aspect the invention provides an aqueous solution of acompound of the invention. The aqueous solution includes a complexbetween a gadolinium (III) ion and an organic ligand comprising onlyoxygen donor atoms coordinating the gadolinium (III) ion. The solutionhas a pM of at least about 15. The solution is about pH 7.4, andincludes about 10 μM of the ligand and about 1 μM of Gd(III). Thecomplex has a solubility in water of at least about 15 mM, morepreferably, at least about 20 mM.

In a third aspect, the invention provides a complex between a gadolinium(III) ion and an organic ligand. The ligand includes the structureaccording to Formula I:

in which, the symbols R¹, R², and R³ are independently selected from alinking member, an aryl group substituent, H, substituted orunsubstituted alkyl, substituted or unsubstituted aryl, substituted orunsubstituted heteroalkyl, hydroxy, carboxy, amide, ester, amine, andreactive functional groups, with the proviso that when A is nitrogen, R¹is other than amino, and with the further proviso that when E isnitrogen, R³ is not present. The symbol R⁴ represents a linking member,H, substituted or unsubstituted alkyl, substituted or unsubstitutedheteroalkyl, substituted or unsubstituted aryl, hydroxy, carboxy, amide,or an ester group. A, E and Z are independently selected from carbon andnitrogen. The gadolinium complex preferably has a solubility in water ofat least about 15 mM, more preferably, at least about 20 mM.

Also provided is a method of performing contrast-enhanced magneticresonance imaging on a patient. The method includes administering to thepatient an amount of a compound of the invention sufficient to providecontrast enhancement, and acquiring a contrast enhanced MR image.

The invention also provides selective structural modifications of theligand scaffold result in in vivo residence times that are enhancedrelevant to analogous compounds that are not modified. Othermodifications are provided that generate greater specificity in the invivo distribution of the modified Gd³⁺ complexes.

Other objects and advantages of the invention will be apparent to thoseof skill in the art from the detailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a collection of poly(amino-carboxylate) ligands used forchelating Gd(III).

FIG. 2 is a diagram of a Gd(III) complex showing inner and outer-spherewater molecules, and the parameters that affect inner-sphere relaxivity.

FIG. 3 is a graphical presentation comparing relaxivity data forcompounds of the invention and Gd(III)-polyaminocarboxylates.

FIG. 4 is a graphical presentation comparing water exchange rates forcompounds of the invention and Gd(III)-polyaminocarboxylates.

FIG. 5 is a species distribution diagram for the Gd³⁺/TREN-1-Me-HOPOsystem, calculated for 1 μM Gd and 10 μM ligand. The ligand isrepresented as “L” in the diagram.

FIG. 6 is a graphical display of the selectivity of TREN-1-Me-HOPO forGd³⁺ over Ca2⁺ and Zn²⁺ as compared with DTPA and DTPA-BMA.

FIG. 7 displays the structure of a representative Gd(III)-HOPO chelateand provides a graphical comparison the the NMRD profile of thiscompound with those of representative Gd(III)-polyaminocarboxylates.

FIG. 8 is an exemplary synthetic route to compounds of the inventionhaving derivatized scaffolds with alkylene moieties of differentlengths.

FIG. 9 is a graphical representation of the variation in τ_(m) with PEGlength.

FIG. 10 is a graphical display of the relaxivities of various complexesof the invention.

FIG. 11 provides exemplary structures that are known to interact withhuman serum albumin.

FIG. 12 displays structures of exemplary protein binding chelates of theinvention.

FIG. 13 is a graphical representation of the variation in the affinityof complexes of the invention for human serum albumin as thehydrophobicity/hydrophilicity of the complex is varied indicating thatthe invention provides a method to systematically vary the degree andstrength of binding of a complex to a serum protein.

FIG. 14 is a graphical representation of the variation in therelatxation of complexes of the invention bound to human serum albuminas the hydrophobicity/hydrophilicity of the complex is varied indicatingthat the invention provides a method to systematically vary therelaxivity of the complex based on the degree and strength of binding ofa complex to a serum protein.

FIG. 15 displays structures of motifs of protein binding complexes ofthe invention.

FIG. 16 is a NMRD profile of 47 in water at 25° C. and pH=7.4. Curvefitted with r=3.1 Å, q=1, τ_(R)=300 ps, τ_(M)=31 ns (from ¹⁷O NMR),τ_(V)=27 ns, Δ²=8.4×10¹⁹, a=4.0 Å, D=2.24×10⁻⁵ cm²s⁻¹.

FIG. 17 is a graphical display of the ¹⁷O NMR transverse relaxationrates as a function of temperature (273-355 K), measured at 2.1 T (90MHz proton Larmor frequency) for 21 mM aqueous solutions of thecomplexes: (a) Gd-TREN-bisHOPO-TAM-Me, pH=7.4, curve fitted with q=2,Δ²=1.9×10²⁰ s⁻², τ_(M)=8±1 ns (at 298 K) and ΔH_(M)=2.4 kJ/mol; (b) 46,pH=7.6, curve fitted with q=1, Δ²=8.0×10¹⁹ s⁻², τ_(M)=19±2 ns (at 298 K)and ΔH_(M)=10 kJ/mol and (c) 47, pH=7.5, curve fitted with q=1,Δ²=7.3×10¹⁹ s⁻², τ_(M)=31±2 ns (at 298 K) and ΔH_(M)=6 kJ/mol.

FIG. 18 displays the calculated relaxivity at 20, 40, 60 and 80 MHz as afunction of τ_(M) for a macromolecular complex (τ_(R)=10 ns). Typicalparameters of DOTA-like Gd(III) complexes were utilized: q=1, r=3.0 Å,Δ²=1.5×10¹⁹ s⁻², τ_(V)=20 ps, a=3.8 Å, D=2.24×10⁻⁵ cm²s⁻¹.

FIG. 19 displays the ¹H longitudinal relaxation rate of H₂O as afunction of HSA concentration. Experiment involved titration of a 0.25mmol/L solution of the 47 with HSA at 20 MHz, 25° C. and pH=6.5.Analysis of data according to the PRE (Proton Relaxation Enhancement)method gives K_(A)=186±50 M⁻¹ and r_(1p)(bound)=74±14 mM⁻¹ s⁻¹. Therelatively weak interaction results in large standard errors.

FIG. 20 displays the temperature dependence of the paramagneticcontribution (R₂) to the transverse ¹⁷O water relaxation rate of 55(left) and 58 (right).

FIG. 21 displays the water proton longitudinal relaxation rate of asolution of complex as a function of HSA concentration: 55 (top left),58 (top right), 48 (bottom)

FIG. 22 displays the 24 h post injection biodistribution of¹⁵³Gd[TREN-bis(HOPO)-(TAM-DME)].

FIG. 23 displays the pGd, pCa and pZn values for various complexes ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

1. Introduction

The present invention provides chelating agents that are useful for manypurposes including, but not limited to, contrast agents for imagingmodalities (e.g., MRI, CT), metal ion decorporation (e.g., iron,actinides), and the like. The chelating agents are also of use forbinding radioisotopes utilized in nuclear medicine, gamma camerascintigraphy, and other medical applications. There is also provided anarray of metal complexes formed between the chelates of the inventionand many metal ions. The chelating agents form surprisingly stable,highly water-soluble complexes with metal ions.

1.1 Definitions

The term, “pGd,” as used herein is −log₁₀[Gd^(III)]_(free) at pH=7.4,and pre-equilibrium concentrations of [L]=10 μM, [Gd^(III)]=1 μM.

“HOPO,” as used herein refers to hydroxypyridonate.

The term “HOPY,” refers to hydroxypyrimidinone

The term “TAM,” refers to dihydroxyterephthalamide.

“Reactive functional group,” as used herein refers to groups including,but not limited to, olefins, acetylenes, alcohols, phenols, ethers,oxides, halides, aldehydes, ketones, carboxylic acids, esters, amides,cyanates, isocyanates, thiocyanates, isothiocyanates, amines,hydrazines, hydrazones, hydrazides, diazo, diazonium, nitro, nitrites,mercaptans, sulfides, disulfides, sulfoxides, sulfones, sulfonic acids,sulfinic acids, acetals, ketals, anhydrides, sulfates, sulfenic acidsisonitriles, amidines, imides, imidates, nitrones, hydroxylamines,oximes, hydroxamic acids thiohydroxamic acids, allenes, ortho esters,sulfites, enamines, ynamines, ureas, pseudoureas, semicarbazides,carbodiimides, carbamates, imines, azides, azo compounds, azoxycompounds, and nitroso compounds. Reactive functional groups alsoinclude those used to prepare bioconjugates, e.g., N-hydroxysuccinimideesters, maleimides and the like. Methods to prepare each of thesefunctional groups are well known in the art and their application to ormodification for a particular purpose is within the ability of one ofskill in the art (see, for example, Sandler and Karo, eds. ORGANICFUNCTIONAL GROUP PREPARATIONS, Academic Press, San Diego, 1989).

“Non-covalent protein binding groups” are moieties that interact with anintact or denatured polypeptide in an associative manner. Theinteraction may be either reversible or irreversible in a biologicalmilieu. The incorporation of a “non-covalent protein binding group“ intoa chelating agent or complex of the invention provides the agent orcomplex with the ability to interact with a polypeptide in anon-covalent manner. Exemplary non-covalent interactions includehydrophobic-hydrophobic and electrostatic interactions. Exemplary“non-covalent protein binding groups” include anionic groups, e.g.,phosphate, thiophosphate, phosphonate, carboxylate, boronate, sulfate,sulfone, thiosulfate, and thiosulfonate.

As used herein, “linking member” refers to a covalent chemical bond thatincludes at least one heteroatom. Exemplary linking members include—C(O)NH—, —C(O)O—, —NH—, —S—, —O—, and the like.

The term “targeting group” is intended to mean a moiety that is: (1)able to actively direct the entity to which it is attached (e.g.,contrast agent) to a target region, e.g., a tumor; or (2) ispreferentially passively absorbed by or entrained within a targettissue, for example a tumor. The targeting group can be a smallmolecule, which is intended to include both non-peptides and peptides.The targeting group can also be a macromolecule, which includes, but isnot limited to, saccharides, lectins, receptors, ligand for receptors,proteins such as BSA, antibodies, poly(ethers), dendrimers, poly(aminoacids) and so forth.

The term “cleavable group” is intended to mean a moiety that allows forrelease of the chelate from the rest of the conjugate by cleaving a bondlinking the chelate (or chelate linker arm construct) to the remainderof the conjugate. Such cleavage is either chemical in nature, orenzymatically mediated. Exemplary enzymatically cleavable groups includenatural amino acids or peptide sequences that end with a natural aminoacid.

In addition to enzymatically cleavable sites, it is within the scope ofthe present invention to include one or more sites that are cleaved bythe action of an agent other than an enzyme. Exemplary non-enzymaticcleavage agents include, but are not limited to, acids, bases, light(e.g., nitrobenzyl derivatives, phenacyl groups, benzoin esters), andheat. Many cleaveable groups are known in the art. See, for example,Jung et al., Biochem. Biophys. Acta, 761: 152-162 (1983); Joshi et al.,J. Biol. Chem., 265: 14518-14525 (1990); Zarling et al., J. Immunol.,124: 913-920 (1980); Bouizar et al., Eur. J. Biochem., 155: 141-147(1986); Park et al., J. Biol. Chem., 261: 205-210 (1986); Browning etal., J. Immunol., 143: 1859-1867 (1989). Moreover a broad range ofcleavable, bifunctional (both homo- and hetero-bifunctional) spacer armsare commercially available from suppliers such as Pierce.

The symbol

, whether utilized as a bond or displayed perpendicular to a bondindicates the point at which the displayed moiety is attached to theremainder of the molecule, solid support, etc.

Certain compounds of the present invention can exist in unsolvated formsas well as solvated forms, including hydrated forms. In general, thesolvated forms are equivalent to unsolvated forms and are encompassedwithin the scope of the present invention. Certain compounds of thepresent invention may exist in multiple crystalline or amorphous forms.In general, all physical forms are equivalent for the uses contemplatedby the present invention and are intended to be within the scope of thepresent invention.

Certain compounds of the present invention possess asymmetric carbonatoms (optical centers) or double bonds; the racemates, diastereomers,geometric isomers and individual isomers are encompassed within thescope of the present invention.

The compounds of the invention may be prepared as a single isomer (e.g.,enantiomer, cis-trans, positional, diastereomer) or as a mixture ofisomers. In a preferred embodiment, the compounds are prepared assubstantially a single isomer. Methods of preparing substantiallyisomerically pure compounds are known in the art. For example,enantiomerically enriched mixtures and pure enantiomeric compounds canbe prepared by using synthetic intermediates that are enantiomericallypure in combination with reactions that either leave the stereochemistryat a chiral center unchanged or result in its complete inversion.Alternatively, the final product or intermediates along the syntheticroute can be resolved into a single stereoisomer. Techniques forinverting or leaving unchanged a particular stereocenter, and those forresolving mixtures of stereoisomers are well known in the art and it iswell within the ability of one of skill in the art to choose andappropriate method for a particular situation. See, generally, Furnisset al. (eds.),VOGEL'S ENCYCLOPEDIA OF PRACTICAL ORGANIC CHEMISTRY 5^(TH)ED., Longman Scientific and Technical Ltd., Essex, 1991, pp. 809-816;and Heller, Acc. Chem. Res. 23: 128 (1990).

The compounds of the present invention may also contain unnaturalproportions of atomic isotopes at one or more of the atoms thatconstitute such compounds. For example, the compounds may beradiolabeled with radioactive isotopes, such as for example tritium(³H), iodine-125 (¹²⁵I) or carbon-14 (¹⁴C). All isotopic variations ofthe compounds of the present invention, whether radioactive or not, areintended to be encompassed within the scope of the present invention.

Where substituent groups are specified by their conventional chemicalformulae, written from left to right, they equally encompass thechemically identical substituents, which would result from writing thestructure from right to left, e.g., —CH₂O— is intended to also recite—OCH₂—.

The term “alkyl,” by itself or as part of another substituent, means,unless otherwise stated, a straight or branched chain, or cyclichydrocarbon radical, or combination thereof, which may be fullysaturated, mono- or polyunsaturated and can include di- and multivalentradicals, having the number of carbon atoms designated (i.e. C₁-C₁₀means one to ten carbons). Examples of saturated hydrocarbon radicalsinclude, but are not limited to, groups such as methyl, ethyl, n-propyl,isopropyl, n-butyl, t-butyl, isobutyl, sec-butyl, cyclohexyl,(cyclohexyl)methyl, cyclopropylmethyl, homologs and isomers of, forexample, n-pentyl, n-hexyl, n-heptyl, n-octyl, and the like. Anunsaturated alkyl group is one having one or more double bonds or triplebonds. Examples of unsaturated alkyl groups include, but are not limitedto, vinyl, 2-propenyl, crotyl, 2-isopentenyl, 2-(butadienyl),2,4-pentadienyl, 3-(1,4-pentadienyl), ethynyl, 1- and 3-propynyl,3-butynyl, and the higher homologs and isomers. The term “alkyl,” unlessotherwise noted, is also meant to include those derivatives of alkyldefined in more detail below, such as “heteroalkyl.” Alkyl groups thatare limited to hydrocarbon groups are termed “homoalkyl”.

The term “alkylene” by itself or as part of another substituent means adivalent radical derived from an alkane, as exemplified, but notlimited, by —CH₂CH₂CH₂CH₂—, and further includes those groups describedbelow as “heteroalkylene.” Typically, an alkyl (or alkylene) group willhave from 1 to 24 carbon atoms, with those groups having 10 or fewercarbon atoms being preferred in the present invention. A “lower alkyl”or “lower alkylene” is a shorter chain alkyl or alkylene group,generally having eight or fewer carbon atoms.

The terms “alkoxy,” “alkylamino” and “alkylthio” (or thioalkoxy) areused in their conventional sense, and refer to those alkyl groupsattached to the remainder of the molecule via an oxygen atom, an aminogroup, or a sulfur atom, respectively.

The term “heteroalkyl,” by itself or in combination with another term,means, unless otherwise stated, a stable straight or branched chain, orcyclic hydrocarbon radical, or combinations thereof, consisting of thestated number of carbon atoms and at least one heteroatom selected fromthe group consisting of O, N, Si and S, and wherein the nitrogen andsulfur atoms may optionally be oxidized and the nitrogen heteroatom mayoptionally be quaternized. The heteroatom(s) O, N and S and Si may beplaced at any interior position of the heteroalkyl group or at theposition at which the alkyl group is attached to the remainder of themolecule. Examples include, but are not limited to, —CH₂—CH₂—O—CH₃,—CH₂—CH₂—NH—CH₃, —CH₂—CH₂—N(CH₃)—CH₃, —CH₂—S—CH₂—CH₃, —CH₂—CH₂,—S(O)—CH₃, —CH₂—CH₂—S(O)₂—CH₃, —CH═CH—O—CH₃, —Si(CH₃)₃, —CH₂—CH═N—OCH₃,and —CH═CH—N(CH₃)—CH₃. Up to two heteroatoms may be consecutive, suchas, for example, —CH₂—NH—OCH₃ and —CH₂—O—Si(CH₃)₃. Similarly, the term“heteroalkylene” by itself or as part of another substituent means adivalent radical derived from heteroalkyl, as exemplified, but notlimited by, —CH₂—CH₂—S—CH₂—CH₂— and —CH₂—S—CH₂—CH₂—NH—CH₂—. Forheteroalkylene groups, heteroatoms can also occupy either or both of thechain termini (e.g., alkyleneoxy, alkylenedioxy, alkyleneamino,alkylenediamino, and the like). Still further, for alkylene andheteroalkylene linking groups, no orientation of the linking group isimplied by the direction in which the formula of the linking group iswritten. For example, the formula —C(O)₂R′— represents both —C(O)₂R′—and —R′C(O)₂—.

The terms “cycloalkyl” and “heterocycloalkyl”, by themselves or incombination with other terms, represent, unless otherwise stated, cyclicversions of “alkyl” and “heteroalkyl”, respectively. Additionally, forheterocycloalkyl, a heteroatom can occupy the position at which theheterocycle is attached to the remainder of the molecule. Examples ofcycloalkyl include, but are not limited to, cyclopentyl, cyclohexyl,1-cyclohexenyl, 3-cyclohexenyl, cycloheptyl, and the like. Examples ofheterocycloalkyl include, but are not limited to,1-(1,2,5,6-tetrahydropyridyl), 1-piperidinyl, 2-piperidinyl,3-piperidinyl, 4-morpholinyl, 3-morpholinyl, tetrahydrofuran-2-yl,tetrahydrofuran-3-yl, tetrahydrothien-2-yl, tetrahydrothien-3-yl,1-piperazinyl, 2-piperazinyl, and the like.

The terms “halo” or “halogen,” by themselves or as part of anothersubstituent, mean, unless otherwise stated, a fluorine, chlorine,bromine, or iodine atom. Additionally, terms such as “haloalkyl,” aremeant to include monohaloalkyl and polyhaloalkyl. For example, the term“halo(C₁-C₄)alkyl” is mean to include, but not be limited to,trifluoromethyl, 2,2,2-trifluoroethyl, 4-chlorobutyl, 3-bromopropyl, andthe like.

The term “aryl” means, unless otherwise stated, a polyunsaturated,aromatic, substituent that can be a single ring or multiple rings(preferably from 1 to 3 rings), which are fused together or linkedcovalently. The term “heteroaryl” refers to aryl groups (or rings) thatcontain from one to four heteroatoms selected from N, O, and S, whereinthe nitrogen and sulfur atoms are optionally oxidized, and the nitrogenatom(s) are optionally quaternized. A heteroaryl group can be attachedto the remainder of the molecule through a heteroatom. Non-limitingexamples of aryl and heteroaryl groups include phenyl, 1-naphthyl,2-naphthyl, 4-biphenyl, 1-pyrrolyl, 2-pyrrolyl, 3-pyrrolyl, 3-pyrazolyl,2-imidazolyl, 4-imidazolyl, pyrazinyl, 2-oxazolyl, 4-oxazolyl,2-phenyl-4-oxazolyl, 5-oxazolyl, 3-isoxazolyl, 4-isoxazolyl,5-isoxazolyl, 2-thiazolyl, 4-thiazolyl, 5-thiazolyl, 2-furyl, 3-furyl,2-thienyl, 3-thienyl, 2-pyridyl, 3-pyridyl, 4-pyridyl, 2-pyrimidyl,4-pyrimidyl, 5-benzothiazolyl, purinyl, 2-benzimidazolyl, 5-indolyl,1-isoquinolyl, 5-isoquinolyl, 2-quinoxalinyl, 5-quinoxalinyl,3-quinolyl, and 6-quinolyl. Substituents for each of the above notedaryl and heteroaryl ring systems are selected from the group ofacceptable substituents described below.

For brevity, the term “aryl” when used in combination with other terms(e.g., aryloxy, arylthioxy, arylalkyl) includes both aryl and heteroarylrings as defined above. Thus, the term “arylalkyl” is meant to includethose radicals in which an aryl group is attached to an alkyl group(e.g., benzyl, phenethyl, pyridylmethyl and the like) including thosealkyl groups in which a carbon atom (e.g., a methylene group) has beenreplaced by, for example, an oxygen atom (e.g., phenoxymethyl,2-pyridyloxymethyl, 3-(1-naphthyloxy)propyl, and the like).

Each of the above terms (e.g., “alkyl,” “heteroalkyl,” “aryl” and“heteroaryl”) are meant to include both substituted and unsubstitutedforms of the indicated radical. Preferred substituents for each type ofradical are provided below.

Substituents for the alkyl and heteroalkyl radicals (including thosegroups often referred to as alkylene, alkenyl, heteroalkylene,heteroalkenyl, alkynyl, cycloalkyl, heterocycloalkyl, cycloalkenyl, andheterocycloalkenyl) are generically referred to as “alkyl groupsubstituents,” and they can be one or more of a variety of groupsselected from, but not limited to: —OR′, ═O, ═NR′, ═N—OR′, —NR′R″, —SR′,-halogen, —SiR′R″R′″, —OC(O)R′, —C(O)R′, —CO₂R′, —CONR′R″, —OC(O)NR′R″,—NR″C(O)R′, —NR′—C(O)NR″R′″, —NR″C(O)₂R′, —NR—C(NR′R″R′″)═NR″″,—NR—C(NR′R″)═NR′″, —S(O)R′, —S(O)₂R′, —S(O)₂NR′R″, —NRSO₂R′, —CN and—NO₂ in a number ranging from zero to (2 m′+1) where m′ is the totalnumber of carbon atoms in such radical. R′, R″, R′″ and R″″ eachpreferably independently refer to hydrogen, substituted or unsubstitutedheteroalkyl, substituted or unsubstituted aryl, e.g., aryl substitutedwith 1-3 halogens, substituted or unsubstituted alkyl, alkoxy orthioalkoxy groups, or arylalkyl groups. When a compound of the inventionincludes more than one R group, for example, each of the R groups isindependently selected as are each R′, R″, R′″ and R″″ groups when morethan one of these groups is present. When R′ and R″ are attached to thesame nitrogen atom, they can be combined with the nitrogen atom to forma 5-, 6-, or 7-membered ring. For example, —NR′R″ is meant to include,but not be limited to, 1-pyrrolidinyl and 4-morpholinyl. From the abovediscussion of substituents, one of skill in the art will understand thatthe term “alkyl” is meant to include groups including carbon atoms boundto groups other than hydrogen groups, such as haloalkyl (e.g., —CF₃ and—CH₂CF₃) and acyl (e.g., —C(O)CH₃, —C(O)CF₃, —C(O)CH₂OCH₃, and thelike).

Similar to the substituents described for the alkyl radical,substituents for the aryl and heteroaryl groups are generically referredto as “aryl group substituents.” The substituents are selected from, forexample: halogen, —OR′, ═O, ═NR′, ═N—OR′, —NR′R″, —SR′, -halogen,—SiR′R″R′″, —OC(O)R′, —C(O)R′, —CO₂R′, —CONR′R−, —OC(O)NR′R″,—NR″C(O)R′, —NR′—C(O)NR″R′″, —NR″C(O)₂R′, —NR—C(NR′R″R′″)═NR″″,—NR—C(NR′R″)═NR′″, —S(O)R′, —S(O)₂R′, —S(O)₂NR′R″, —NRSO₂R′, —CN and—NO₂, —R′, —N₃, —CH(Ph)₂, fluoro(C₁-C₄)alkoxy, and fluoro(C₁-C₄)alkyl,in a number ranging from zero to the total number of open valences onthe aromatic ring system; and where R′, R″, R′″ and R″″ are preferablyindependently selected from hydrogen, substituted or unsubstitutedalkyl, substituted or unsubstituted heteroalkyl, substituted orunsubstituted aryl and substituted or unsubstituted heteroaryl. When acompound of the invention includes more than one R group, for example,each of the R groups is independently selected as are each R′, R″, R′″and R″″ groups when more than one of these groups is present.

Two of the substituents on adjacent atoms of the aryl or heteroaryl ringmay optionally be replaced with a substituent of the formula-T-C(O)—(CRR′)_(q)—U—, wherein T and U are independently —NR—, —O—,—CRR′— or a single bond, and q is an integer of from 0 to 3.Alternatively, two of the substituents on adjacent atoms of the aryl orheteroaryl ring may optionally be replaced with a substituent of theformula -A-(CH₂)_(r)—B—, wherein A and B are independently —CRR′—, —O—,—NR—, —S—, —S(O)—, —S(O)₂—, —S(O)₂NR′— or a single bond, and r is aninteger of from 1 to 4. One of the single bonds of the new ring soformed may optionally be replaced with a double bond. Alternatively, twoof the substituents on adjacent atoms of the aryl or heteroaryl ring mayoptionally be replaced with a substituent of the formula—(CRR′)_(s)—X—(CR″R′″)_(d)—, where s and d are independently integers offrom 0 to 3, and X is —O—, —NR′—, —S—, —S(O)—, —S(O)₂—, or —S(O)₂NR′—.The substituents R, R′, R″ and R′″ are preferably independently selectedfrom hydrogen or substituted or unsubstituted (C₁-C₆)alkyl.

As used herein, the term “heteroatom” is meant to include oxygen (O),nitrogen (N), sulfur (S) and silicon (Si).

A “MRI signal generating moiety” is a species that affects T1 or T2 ofwater molecules in a sample or subject undergoing a MRI experiment.

The broken line as a component of a chemical structure implies that theposition of the bond may be varied, such as within a ring structure, orthat the bond may be either present or absent, such that the principlesof chemical valency are obeyed.

Chelating Agents as MRI Contrast Agents

The image intensity in MRI, largely composed of the NMR signal of waterprotons (¹H), is the result of a complex interplay of numerous factors.These include the longitudinal (T₁) and transverse (T₂) protonrelaxation times, proton density of the imaged tissues and instrumentalparameters (such as magnetic field strength) (Aime, S. B. et al., Coord.Chem. Rev. 1999, 185-6, 321). Contrast agents (complexes of paramagneticions) decrease the relaxation times of nearby water protons by dipolarinteractions, resulting in enhanced signal intensity of the tissuecontaining the agent (Lauffer, R. B., Chem. Rev. 1987, 87, 901). Thehigh magnetic moment and relatively long electronic relaxation time ofGd(III) make it ideally suited as the active component in MRI contrastagents (Banci, L. B. et al., C. Nuclear and Electronic Relaxation; VCH:Weinheim, 1991). Images that are acquired with the aid of a Gd(III)contrast agent are preferably visualized as “Ti-weighted images” sincethe percentage change in 1/T₁, in tissue is much greater than that in1/T₂ (Caravan, P. E. et al., Chem. Rev. 1999, 99, 2293).

In one aspect, the present invention relates to novel MRI contrastagents and compositions of novel MRI contrasting agents with fasterwater exchange rates (i.e. with τ_(m) and τ_(r) preferably in the low nsregime) and desirable in vivo persistence. These novel agents facilitatediagnosis of physiological abnormalities in specific regions of the bodyover longer time periods than are currently possible.

Although the discussion below focuses on gadolinium as a representativeparamagnetic ion that is coordinated by the chelating agents of theinvention, it will be apparent to those of skill in the art that thechelating agents of the invention are appropriate for complexing manymetal ions. Thus, it is within the scope of the present invention toutilize ions of, e.g., transition metals, and lanthanides other thangadolinium. Exemplary metal ions include the ions of Dy, Fe, Mn, Pu, andU.

For a metal chelate to be useful as a contrast enhancing agent in MRI,the agent must satisfy several requirements. Broadly stated the threemost important requirements are stability, relaxivity and watersolubility, each of which must high values. The inventors haverecognized that ligand systems that have only oxygen donor atoms providecomplexes of Gd(III) that exhibit excellent stability, have highrelaxivity and are highly water soluble. Each of the essentialcharacteristics of a useful complex between Gd(III) and a ligand withonly oxygen donor atoms is discussed below.

Water Solubility

As with most diagnostic agents, it is desired that a MRI contrastenhancing agent be as highly water soluble as possible. The watersolubility of MRI contrast agents is of particular importance becausethe agents are administered in multigram dosages to the subject of theimaging experiment. A highly water-soluble agent requires a lowerinjection volume for administration. Lower injection volume correlateswith ease of agent administration and decreased patient discomfort.

The inventors have recognized that for a complex of Gd(III) with aligand having only oxygen donor atoms to be a promising candidate for aMRI contrast agent, the complex preferably has a water solubility of atleast about 15 mM and, more preferably, at least about 20 mM. Thepresent invention provides complexes between Gd(III) and ligands withonly oxygen donors that meet and/or exceed this requirement. Theinvention provides Gd(III) complexes as described above, having watersolubilities of from about 50 mM to about 300 mM and, preferably fromabout 100 mM to about 1 M.

In contrast to the compounds of the present invention, the compoundsknown in the art are relatively water insoluble. For example,TREN-MOE-3,2-HOPO has a water solubility at pH 7.4 of about 1 mM. Incontrast, a compound of the present invention such as the Gd(III)complex of TREN-HOPY has a relaxivity of at least about 100 mM.Moreover, the present invention provides poly(ether) conjugates ofhomopodal and heteropodal ligands having water solubilities that aredramatically improved over compounds reported to date. Exemplarysolubilities of compounds of the invention and known compounds arecompared below in Table 1.

TABLE 1 Complex Water Solubility (pH 7, 25° C.) Gd-TREN-HOPO (7) 0.1 mMGd-TREN-HOPO-MOE 1 mM Gd-TRENGly-HOPO (7A) 0.24 mM (ε = 21,000lmol⁻¹cm⁻¹) Gd-TREN-HOPY 100 mM Gd-TREN-bisHOPO- <3 mM (TAM-Me)Gd-SerTREN-HOPO 15 mM Gd-TREN-bisHOPO- 300 mM* (TAM-PEG5000) (47)Gd-TREN-bis(HOPO-Bn)- 26.8 mM* (ε = 17,443 lmol⁻¹cm⁻¹) (TAM-PEG550) (55)Gd-TREN-bisHOPO- 48.3 mM* (ε = 17,620 lmol⁻¹cm⁻¹) (TAM-PEG450) (48)Gd-TREN-bisHOPO- 20.1 mM* (ε = 17,641 lmol⁻¹cm⁻¹) HOPO = 1-Me-3,2-HOPOor 6-Me-3,2-HOPO *minimum solubility in H₂O (0.01 M HEPES).Relaxivity

The potency of a MRI contrast agent is generally given in terms of themagnitude of its relaxivity. The relaxivity, r₁, of a MRI contrastagent, as used herein, refers to the amount of increase in 1/T₁ signalintensity that occurs per millimolar of Gd(III). When considering theinteractions of water molecules with the contrast agent on the atomicscale, the relaxivity can be sub-divided into inner sphere and outersphere contributions (FIG. 2) (Lauffer, R. B., Chem. Rev. 1987, 87,901).

A mechanism underlying the enhanced relaxivity of the complexes of theinvention is the near ideal time-scale of the water exchange kinetics ofthe complexes. The inventors have recognized that for a complex formedbetween a Gd(III) ion and a ligand with only oxygen donor atoms to be acandidate MR imaging agent candidate, the metal ion of the complexpreferably has a water exchange rate of at least about 10×10⁶ sec⁻¹(FIG. 3).

While the clinically approved gadolinium(III) chelates are alloctadentate with one coordinated water molecule (q=1) (Caravan, P. E. etal., Chem. Rev. 1999, 99, 2293), hexadentate tripodal hydroxypyridonatecomplexes such as TREN-1-Me-HOPO form stable, di-aquo complexes withGd³⁺ (q=2) (Xu, J. et al., J. Am. Chem. Soc. 1995, 11 7, 7245; Johnson,A. R. et al., Inorg. Chem. 2000, 39, 2652-2660). The relaxivity of thecomplexes of the invention is typically at least two-fold greater thanthat of agents based upon the polyaminocarboxylate motif (FIG. 3).

For example, the relaxivity (r₁) of TREN-1-Me-HOPO at 37° C. and 20 MHzis 10.5 mM⁻¹s⁻¹, some 2.5 times that of [Gd(DTPA)(H₂O)]. This is partlydue to the higher number of coordinated water molecules (q=2) as well asthe increased molecular weight (which results in a longer rotationalcorrelation time, τ_(r)). Another factor that enhances the relaxivity ofthe complexes of the invention relative to the polyaminocarboxylates isthe nearly ideal time-scale of the water exchange kinetics of thepresent complexes (FIG. 4). Another advantageous feature ofTREN-1-Me-HOPO for clinical applications is its neutrality, whichreduces osmolality effects in vivo, therefore lessening the discomfortof patients upon its intravenous administration (Lauffer, R. B., Chem.Rev. 1987, 87, 901).

The relaxivity in water of the complexes of the invention is preferablygreater than about 5 mM⁻¹s⁻¹, more preferably greater than about 6mM⁻¹s⁻¹. A presently preferred range of relaxivities of compounds of theinvention is from about 6 mM⁻¹s⁻¹ to about 15 mM⁻¹s⁻¹.

Stability

Since free Gd(III) is toxic it must be bound by a multidentate ligand toform a complex of high stability before it can be safely administered topatients. Several groups have investigated the relationship between thechemical properties of Gd(III) complexes and their toxicity (Cacheris,W. P. Q. et al., Magn. Reson. Imag. 1990, 8, 467; Wedeking, P. K. etal., Magn. Reson. Imag. 1991, 10, 641; Pattagunta, N. R. G. et al.,Invest. Radiol. 1996, 10, 619; Pattagunta, N. R. G. et al., Invest.Radiol. 1996, 12, 739; Kumar, K. T. et al., Inorg Chem. 1995, 34, 6472).The toxicity is due to the exchange of Gd³⁺ with physiological cationssuch as Zn(II), Cu(II), and Ca(II)—the free ligand and Gd(III) ion aresignificantly more harmful than the intact complex (Lauffer, R. B.,Chem. Rev. 1987, 87, 901).

Similar to other MRI contrast agents, a complex between Gd(III) and aligand with only oxygen donor atoms must be acceptably stable andnon-toxic. The inventors have recognized that a complex between Gd(III)and an oxygen donor ligand will preferably form an aqueous solution thathas a pGd of at least about 18 when the aqueous solution is pH about 7.4and includes about 10 μM of ligand and about 1 μM of Gd. See, Equation4, infra.

The speciation of an exemplary complex of the invention,Gd(III)-TREN-1-Me-HOPO in water is unchanged over the pH range 5.5-10(FIG. 5), indicating that at physiological pH the ligand remainsdeprotonated and the complex remains intact. TREN-1-Me-HOPO is alsohighly thermodynamically stable (pGd=20.3) and is of very low toxicity.In comparison with DTPA, DTPA-BMA, and DOTA, TREN-1-Me-HOPO exhibitsenhanced selectivity (FIG. 6) for Gd³⁺ over the physiological metal ionsCa²⁺ and Zn²⁺ (Xu, J. et al., J. Am. Chem. Soc. 1995, 117, 7245), whichindicates that the dissociation of TREN-1-Me-HOPO is minimal in vivo.The low toxicity of TREN-1-Me-HOPO to mice is also of pre-clinicalimportance (Xu, J. et al., J. Med. Chem. 1995, 38, 2606-2614; Raymond,K. N. et al., U.S. Pat. No. 5,892,029 (1999)).

Complexes

In a first aspect, the present invention provides a complex between agadolinium (III) ion and an organic ligand comprising only oxygen donoratoms coordinating the gadolinium (III) ion. The complexed gadoliniumion has a water exchange rate of at least about 10×10⁶ sec⁻¹. Thesolubility in water of the complex is at least about 15, preferably, atleast about 20 mM.

In an exemplary embodiment, the present invention provides a complexbetween a gadolinium (III) ion and an organic ligand. The ligandincludes a structure according to Formula I:

in which, the symbols R¹, R², and R³ are independently selected from alinking member, an aryl group substituent, H, substituted orunsubstituted alkyl, substituted or unsubstituted aryl, substituted orunsubstituted heteroalkyl, hydroxy, carboxy, amide, ester, and aminogroups, with the proviso that when A is nitrogen, R¹ is other thanamino, and with the further proviso that when E is nitrogen, R³ is notpresent. The symbol R⁴ is a linking member, alkyl group substituent, H,substituted or unsubstituted alkyl, substituted or unsubstitutedheteroalkyl, substituted or unsubstituted aryl, hydroxy, carboxy, amide,or an ester group. A, E and Z are independently selected from carbon andnitrogen. The gadolinium complex has a solubility in water of at leastabout 15 mM, more preferably, at least about 20 mM.

In another exemplary embodiment, the moiety according to Formula I hasthe structure:

in which the symbols R¹ and R⁴ represent members independently selectedfrom a linking member, H, substituted or unsubstituted alkyl,substituted or unsubstituted aryl, substituted or unsubstitutedheteroalkyl, hydroxy, carboxy, amide, or an ester groups. R² and R³ areindependently selected from a linking member, an aryl substituent, H,substituted or unsubstituted alkyl, substituted or unsubstituted aryl,substituted or unsubstituted heteroalkyl, hydroxy, carboxy, amide,ester, or an amino group.

In yet another embodiment according to Formula Ia, R¹ and R² areindependently C₁-C₄ substituted or unsubstituted alkyl, e.g., methyl. Ina further embodiment, R¹ and R² are independently selected from methyland H; and R³ and R⁴ are H. In yet another embodiment, R¹ is selectedfrom substituted or unsubstituted aryl and substituted or unsubstituted(alkyl)aryl; and R² is substituted or unsubstituted C₁-C₄ alkyl.

In another exemplary embodiment, the moiety according to Formula I hasthe structure:

in which R¹ and R⁴ are independently selected from a linking member, H,substituted or unsubstituted alkyl, substituted or unsubstituted aryl,substituted or unsubstituted heteroalkyl, hydroxy, carboxy, amide, andester groups. The symbol R² represents a linking member, an aryl groupsubstituent, H, substituted or unsubstituted alkyl, substituted orunsubstituted aryl, substituted or unsubstituted heteroalkyl, hydroxy,carboxy, amide, ester, and amino groups.

In another embodiment wherein the structure according to Formula I hasthe structure set forth above, the symbols R¹, R² and R⁴ representmembers independently selected from H and substituted or unsubstitutedC₁-C₄ alkyl. In a further embodiment, at least one of R¹ and R² ismethyl.

In yet a further exemplary embodiment, the structure according toFormula I is:

The identities of the radicals R¹—R⁴ are substantially as discussedabove for Formula I.

In another exemplary embodiment, the invention provides a complex inwhich the structure according to Formula I is:

wherein R⁵ is substituted or unsubstituted alkyl, substituted orunsubstituted heteroalkyl, or substituted or unsubstituted aryl. R⁵ isoptionally substituted with one or more organic ligands that are alsocomplexed to the gadolinium (III) ion. The identity of the remainingradicals is substantially as discussed above for Formula I. R⁵ isexemplary of a moiety referred to herein as a “scaffold.” The scaffoldis the backbone that tethers together two or more chelating units toform the ligands of the invention. The scaffold is optionally furthersubstituted with a reactive functional group. The functional groups canbe used to attach the ligand to another species, e.g., a targetingmoiety, polymer, etc.

A further exemplary complex of the invention has a structure accordingto Formula II:

wherein each R¹, R², R³ and R⁴; and each A, E and Z for each of thethree ring systems is independently selected, and the identity of theradicals is as set forth above for Formula I. Thus, for example, a firstR¹ on a first chelating ring structure may be methyl; a second R¹, whichis located on a second ring is ethyl; and a third R¹, which is locatedon a third ring may be benzyl.

In yet a further exemplary embodiment, the invention provides a complexwherein the structure according to Formula II is:

The symbol p represents an integer from 1 to 3.

In yet another exemplary embodiment, the invention provides a complex inwhich the structure according to Formula II is:

In Formulae Ia and IIb, when p is less than three the valency of thenitrogen outside of the parentheses is satisfied by its substitutionwith a substituted or unsubstituted alkyl or substituted orunsubstituted heteroalkyl group. The preferred substituents for thealkyl or heteroalkyl group is a chelating moiety having a structuredifferent than that within the parentheses. When p is greater than one,each of the radicals on each of the p rings is independently selected.The identities of the substituents are as described for Formula II.

In a still further exemplary embodiment, the invention provides acomplex in which the structure according to Formula II is:

The identity of each of the substituents is substantially similar tothose set forth above for Formula II. In a further embodiment accordingto Formula IIc, R¹ is methyl or substituted or unsubstituted benzyl.When the benzyl is substituted, in an exemplary embodiment, it issubstituted with an alkoxy group. In yet another embodiment, at leastone member selected from R¹ and R² is substituted or unsubstituted C₁-C₄alkyl, e.g., as methyl. In another embodiment, at least one R¹ isselected from methyl and polyether. The symbol q represents an integerfrom 1 to 2.

In yet another embodiment, the invention provides a complex wherein thestructure according to Formula II is:

in which the radicals are each independently selected and theiridentities are substantially as discussed above. The symbol q representsan integer from 1 to 2.

In Formulae IIc and IId, when q is less than three, the valency of thenitrogen outside of the parentheses is satisfied by its substitutionwith a substituted or unsubstituted alkyl or substituted orunsubstituted heteroalkyl group. The preferred substituents for thealkyl or heteroalkyl group is a chelating moiety having a structuredifferent than that within the parentheses. When q is greater than one,each of the radicals on each of the p rings is independently selected.The identities of the substituents are as described for Formula II.

Synthesis

The invention provides methods for preparing oxygen donor ligands havingthe have the desirable properties discussed herein. The synthesis ofhomopodal ligands of the invention is exemplified by the synthesis ofGd(TREN-1-Me-HOPO)(H₂O)₂] (tris[(3-hydroxy-1-methyl-2-oxo-1,2-didehydropyridine-4-carboxamido)ethyl]amine),which is the first member of a series of heterocyclic (pyridonate)oxygen donor complexes having characteristics which are desirable in anMRI agent (Xu, J. et al., J. Am. Chem. Soc. 1995, 117, 7245). Thesynthesis of TREN-1-Me-HOPO is shown in Scheme 1.

In Scheme 1, N-methyl-hydroxypyridinone precursor 1 is carboxylated toform the corresponding acid 2. The hydroxyl group of the acid isprotected as the benzyl adduct 3, which is converted to thecorresponding thiazolide 4. The thiazolide is combined with the TRENbackbone, forming 5, which is deprotected by catalytic hydrogenation toafford 6. The gadolinium complex 7, is formed by combining the ligandwith a source of gadolinium ion in the presence of a base.

Those of skill in the art will understand that the method set forthabove is general for the preparation of ligands based upon otherbackbone motifs. Moreover, the method is useful for preparing ligands inwhich the chelating units within a ligand have a structure other thanthe parent HOPO chelating unit. Furthermore, the N-methyl precursor isreadily replaced with its unsubstituted analogue, or by analogues thatare N-substituted with a moiety other than a methyl group, such as asubstituted or unsubstituted alkyl, substituted or unsubstitutedheteroalkyl or a substituted or unsubstituted aryl moiety.

The endocyclic nitrogen of the pyridinone unit is readily functionalizedby methods such as that set forth in Scheme 2:

The protected hydroxypyridinone carboxylic acid ester 8 is a versatileintermediate for the preparation of variously substituted chelatingunits. Thus, in one route, 8 is benzylated at each available site,affording compound 10. In an alternate route, the endocyclic amine isleft unbenzylated, providing compound 9.

The ester of compound 9 is hydrolyzed to the corresponding carboxylicacid 11, which is converted to thiazolide 12. The thiazolide isconjugated with the amine backbone to yield 13, which is subsequentlydebenzylated to compound 14. The Gd(III) complex 15 is formed bystandard methods.

Similarly, compound 10 is coupled with the amine backbone, debenzylatedto yield the hydroxyl moiety and gadolinium chelate 18 is formed. Theroute also provides for forming heteropodal ligands, such as 19, andligands in which the endocyclic nitrogen is functionalized with groupsother than benzyl, e.g,. 20. [Gd(TREN-MOE-HOPO)(H₂O)₂] is arepresentative embodiment of a ligand that is N-substituted with amoiety other than a methyl group, the ligand of the invention includes aHOPO chelating moiety, which is functionalized at the endocyclicnitrogen of the pyridinone moiety with a linker or other group, e.g.,(Johnson, A. R. et al., Inorg. Chem. 2000, 39, 2652-2660). The MOEderivative is more than 10-times more water soluble than the parentcomplex. Moreover, the stability of [Gd(TREN-MOE-HOPO)(H₂O)₂] wassimilar to that of the parent methyl derivative (pGd=19.8).

Ligands Incorporating Terephthalamides

In addition to the methods provided herein for functionalizing thescaffold and the pyridine nitrogen of the chelates of the invention,there is provided an additional method of functionalizing the chelatesfor subsequent attachment to another species or to obtain desirableproperties. The method exploits free amide functionality of a TAM moietyattached to the scaffold. The TAM moiety can be functionalized eitherprior to or following its conjugation with the scaffold. A route to anexemplary compound of the invention, combining HOPO and TAM chelatingmoieties with a TREN backbone is shown in Scheme 3.

In Scheme 3, TREN backbone 21 is contacted with protected thiazolide 22to produce disubstituted TREN 24 with a single free primary amine group.Compound 24 is contacted with the protected di-thiazolide TAM derivative23, yielding activated heteropodal ligand 25 which can be subsequentlyreacted with an amine to form the functionalized heteropodal ligand 27.

In an alternative route, the protected di-thiazolide 23 isfunctionalized with an amine, producing amide 26, prior to its reactionwith the disubstituted TREN derivative 24, producing heteropodal ligand27.

TREN-HOPO-TAM compounds of the invention are readily synthesized usingeither of the routes of Scheme 3. The use of benzyl (Bn) protectinggroups on TAM is generally preferable to the methyl groups previouslyreported (Cohen, S. M. et al., Inorg. Chem. 2000, 39, 4339), since thedeprotection conditions are less severe, making the method amenable touse with a greater range of primary amines (RNH₂). Optimization of thereaction conditions for a particular amine (RNH₂) is well within theabilities of one of skill in the art.

Other examples of mixed ligands are set forth below:

See, for example, Cohen, S. M. et al., Inorg. Chem. 39: 5747 (2000). Thegroups labeled R and R′ are typically H, a substituted or unsubstitutedalkyl, or substituted or unsubstituted heteroalkyl group. In anexemplary embodiment, R and/or R′ are groups that include an ether or areactive functional group.

In yet another exemplary embodiment, the invention provides a compoundaccording to Formula I, wherein at least one of R¹, R², R³ and R⁴ is alinker arm, including a reactive functional group that allows thecompound to be tethered to another species. Alternatively, at least oneof R¹, R², R³ and R⁴ is a moiety that alters a property, e.g., watersolubility, molecular weight, rotational correlation time, complexstability, etc., of the parent compound to which it is affixed.

In a representative embodiment, at least one of R¹, R², R³ and R⁴ hasthe structure:

wherein Z¹ is a member selected from H, CH₂, OR¹⁰, SR¹⁰, NHR¹⁰, OCOR¹¹,OC(O)NHR¹¹, NHC(O)OR¹⁰, OS(O)₂OR¹⁰, and C(O)R¹¹. The symbol R¹⁰represents H, substituted or unsubstituted alkyl, or substituted orunsubstituted heteroalkyl. R¹¹ is a member selected from H, OR¹²,NR¹²NH₂, SH, C(O)R₂, NR¹²H substituted or unsubstituted alkyl andsubstituted or unsubstituted heteroalkyl. R¹² is a member selected fromH, substituted or unsubstituted alkyl and substituted or unsubstitutedalkyl. The symbol X represents a member selected from CH₂, O, S andNR¹³, wherein R¹³ is a member selected from H, substituted orunsubstituted alkyl and substituted or unsubstituted heteroalkyl; andthe symbols j an k represent members independently selected from thegroup consisting of integers from 1 to 20. Other linking moieties usefulwith the compounds of the invention will be apparent to those of skillin the art.

In another exemplary embodiment, at least one of R¹, R², R³ and R⁴ is anether or a polyether, preferably a member selected from ethylene glycol,and ethylene glycol oligomers, having a molecular weight of from about60 daltons to about 10,000 daltons, and more preferably of from about100 daltons to about 1,000 daltons.

Representative polyether-based substituents include, but are not limitedto, the following structures:

in which b is preferably a number from 1 to 100, inclusive. Otherfunctionalized polyethers are known to those of skill in the art, andmany are commercially available from, for example, Shearwater Polymers,Inc. (Alabama).

In a preferred embodiment, the linker includes a reactive functionalgroup for conjugating the compound to another molecule or to a surface.Representative useful reactive groups are discussed in greater detail insucceeding sections. Additional information on useful reactive groups isknown to those of skill in the art. See, for example, Hermanson,BIOCONJUGATE TECHNIQUES, Academic Press, San Diego, 1996.

In yet another exemplary embodiment, the moiety attached to at least oneof R¹, R², R³ and R⁴ combines characteristics of one or more of theabove-recited groups. For example, one preferred linker group combinesboth the attributes of a polyether and a reactive group:

in which j is an integer between 1 and 100, inclusive. Other“bifunctional” linker groups include, but are not limited to, moietiessuch as sugars (e.g., polyol with reactive hydroxyl), amino acids, aminoalcohols, carboxy alcohols, amino thiols, and the like.

The linkers of use in the compounds of the invention can also include acleaveable group. In an exemplary embodiment, the cleaveable group isinterposed between the signal-generating moiety, i.e., the complex, anda targeting agent or macromolecular backbone.

Synthesis and Properties of6-Carboxamido-2,3-methyl-5,4-hydroxypyrimidinone Ligands

The invention also provides complexes based upon a new heterocyclicligand system, hydrogen bond stabilized, bidentate oxo-hydroxy donorligands. In an exemplary embodiment, the new heterocyclic ligand systemutilizes the pyrimidinone analogue of the TREN-HOPO class of ligands,TREN-HOPY (HOPY=6-carboxamido-2,3-methyl-5,4-hydroxypyrimidinone). ThepKa of TREN-HOPY is 6.37, close to that of TREN-1-Me-HOPO (6.12),indicating that the HOPY subunit is slightly more basic. However, thepGd of [Gd(TREN-HOPY)(H₂O)₂] is an acceptable 18.0. Of equal importance,the selectivity of Gd(III)TREN-HOPY over physiological cations Ca(II)and Zn(II) is similar to that of TREN-1-Me-HOPO.

The water solubility of [Gd(TREN-HOPY)(H₂O)₂] is greater than 0.1 M,which is much higher than that of the other HOPOs and is an unexpectedfeature of this complex. The relaxivity of [Gd(TREN-HOPY)(H₂O)₂] inwater at 20 MHz, 25° C. and pH 7.2 is 9.0 mM⁻¹s⁻¹, comparable to[Gd(TREN-HOPO)(H₂O)₂] and significantly higher than commercial MRIagents. The short water residence time of 2 ns, at 298 K, is comparableto members of the HOPO and heteropodand series and indicates that therelaxivity of the HOPY complexes is not limited by slow water exchangekinetics. The rotational correlation time of [Gd(TREN-HOPY)(H₂O)₂] isabout 50% higher than that for the DOTA and DTPA Gd(III) complexes.

In a preferred embodiment, the relaxivity of a chelate such as[Gd(TREN-HOPY)(H₂O)₂] in the presence of endogenous anions isessentially constant, generally indicating that the integrity of thecoordination sphere is maintained and the two water molecules are notdisplaced.

The relaxivity of [Gd(TREN-HOPY)(H₂O)₂] in serum, at 298 K, issignificantly higher (ca. 35% at 20 MHz) than in pure water due to aweak interaction (Ka ca. 100 M⁻¹) of the complex with HSA (resulting ina higher τ_(r)) These highly desirable properties make[Gd(TREN-HOPY)(H₂O)₂] a promising candidate for the preparation ofmacromolecular MRI agents with enhanced relaxivity.

Synthesis of Compounds with the HOPY Motif

Syntheses of compounds with the HOPY motif have been previously reported(Culbertson, T. P., J. Heterocyclic Chem., 16:1423 (1979); Golankiewicz,K.; Wyrzykiewicz, E., Roc. Chem. Ann. Soc. Chim. Pol., 47:1965 (1973);Budesinsky et al., J. Coll. Czec. Chem. Comm., 27:2550 (1963)), albeitas unexpected products in the study of other heterocyclic systems.Difficulties with reproducibility, (Budesinsky et al., J. Coll. Czec.Chem. Comm., 27:2550 (1963)) preparative scale-up, (Golankiewicz, K.;Wyrzykiewicz, E., Roc. Chem. Ann. Soc. Chim. Pol., 47:1965 (1973)) orsubstrate specificity (which precluded the development of a generalsynthesis), (Culbertson, T. P., J. Heterocyclic Chem., 16:1423 (1979);Golankiewicz, K.; Wyrzykiewicz, E., Roc. Chem. Ann. Soc. Chim. Pol.,47:1965 (1973)) highlights the need for a new synthetic route.

In the present invention, synthesis of the desired class offunctionalized HOPYs is achieved by modifying and extending the5,4-hydroxypyrimidinone synthesis of Davoll and Laney, (Davoll, J.;Laney, D. H., J. Chem. Soc., 2124 (1956)) where 28 was reported (Scheme4).

The self-condensation of tetrahydropyran protected ethyl glycolate(Davoll, J.; Laney, D. H., J. Chem. Soc., 2124 (1956)) provides thecorresponding β-ketoester, which is combined in situ with acetamidine inethanol to afford 28. The ¹H and ¹³C NMR spectra of 28 indicates that itis an equal mixture of two diastereomers, resulting from chirality atthe two THP acetal carbons. Methylation of 28 with NaH/MeI/DMF (Jonak etal., J. Org. Chem., 35: 2512 (1970)) gives 30 as a thick oil of >90%purity. Deprotection of 30 (4M HCl dioxane/2-propanol) provides pure 31HCl, alleviating the necessity to further purify 31, the oily precursor.Re-protection of the 5-hydroxy group (BnCl/K₂CO₃/DMF) gave 32.

Compound 32 is a useful intermediate in the preparation of multidentateHOPY-containing ligands where the electronic and structural influencesof an amide group are not desired.

Oxidation of 32 using bleach with TEMPO catalyst under phase transferconditions (Anelli et al., J. Org. Chem., 52:2559 (1987)) providedanalytically pure 33 after careful acidification of the crude reactionextracts. In an exemplary embodiment, the present invention provides asynthetic route of HOPY-containing chelates, which does not requirechromatography in preparing the key ligand precursor, HOPY carboxylicacid, 33.

A second preparation of 33 was also developed as outlined in Scheme 5.

The Claisen condensation product of ethyl oxalate and 2-benzyloxy benzylacetate was reacted with acetamidine in ethanol to provide 34.N-Methylation was performed by the previously described method(MeI/NaH/DMF) and provided the N³ alkylated isomer 35, which, afterhydrolysis gave the HOPY acid 33.

In another exemplary embodiment (Scheme 6), 37 (and then 38) wasprepared from 36 (from Scheme 5).

Conversion of 36 into an activated ester with CDI and in situ reactionwith the appropriate amines, gave 37a-c in 70-90% yield (Scheme 6).Deprotection of the benzyl protecting group and recrystallizationafforded the free ligands 38 which were obtained as analytically purewhite solids. TrenHOPY, 38a, N-EtHOPY, 38b, and N-Me₂HOPY, 38c, wereprepared in this manner.

According to the invention, a ligand can include only HOPY chelatingmoieties attached to a selected scaffold. Alternatively, the HOPYsubunits are combined with HOPO and/or TAM chelating units on a selectedbackbone to provide an array of mixed ligands. Moreover, similar to theHOPO subunits, the HOPY subunits are optionally derivatized with one ormore linker arms or reactive functional groups.

Scaffolds

Throughout the instant specification, the complexes of the invention areexemplified by embodiments in which one or more heterocyclic complexinggroup is attached to a linear, polyfunctional scaffold, forming achelating agent with the correct geometry to complex a metal ion.Generally, scaffolds of use in the complexes and chelating agents of theinvention are exemplified by the use of TREN. The focus on the TRENscaffold is for clarity of illustration only and should not beinterpreted as limiting the scope of the invention to a genus ofchelating agents and complexes having a TREN backbone. Those of skill inthe art will appreciate that a wide array of scaffold structures can beused as scaffold moieties in the compounds of the invention. Forexample, scaffolds of use in the present invention can be linear,cyclic, saturated or unsaturated species. Some exemplary scaffoldmoieties are set forth below:

Additionally, it is within the scope of the present invention to utilizescaffolds that are functionalized with moieties that are available forinteraction with a group on another molecule. Thus, the scaffolds caninclude reactive functional groups, infra, in addition to those that areused to form the link between the scaffold and the chelatingheterocyclic rings.

Functionalization of the TREN Scaffold

In another exemplary embodiment, the water solubility of the complexesof the invention is enhanced by the functionalization of the scaffoldwith an appropriate group. Thus, synthetic methodologies were developedto enhance the solubility of TREN-1-Me-HOPO by the functionalization ofthe TREN cap to form homochiral tris(2-hydroxymethyl)-TREN-1-Me-HOPO(Hajela, S. B. et al., J. Am. Chem. Soc. 2000, 122, 11228).

In an exemplary embodiment, the invention provides a Gd(III) complex of[Gd(SerTREN-1-Me-HOPO)(H₂O)₂] (FIG. 7). The complex has a solubility inwater of ca. 15 mM (25° C., pH 7), a proton relaxivity (r₁) of 9.0mM⁻¹s⁻¹ (at 25 ° C., 20 MHz), which is significantly higher than thoseof the current mono-aquo MRI contrast agents. Of importance, the waterresidence lifetime (τ_(m)) is extremely short (14 ns) and is at leasttwo orders of magnitude higher than commercial MRI agents; this waterexchange rate is close to optimal for slowly rotating MRI contrastagents. Since the Gd(III) cation is 8-coordinate in the ground state, anassociative mechanism is expected to account for the water exchangemechanism (Helm, L. et al., Coord. Chem. Rev. 1999, 187, 151). The fastwater exchange observed is attributed to the small difference in energybetween the eight- and nine-coordinate states. Thus, unlike derivativesof DOTA and DTPA, the relaxivity of derivatives of TREN-1-Me-HOPO arenot limited by slow water exchange.

As illustrated in FIG. 8, the substituent on the scaffold can besubstantially any group including, but not limited to, reactivefunctional groups, substituted or unsubstituted alkyl, substituted orunsubstituted heteroalkyl or substituted or unsubstituted aryl groups.

In yet another exemplary embodiment, the ligand backbone includes anether or poly(ether), preferably an ethylene glycol oligomer. Asynthetic scheme to an exemplary ether derivatized backbone is set forthin Scheme 7:

In Scheme 7, protected tyrosine derivative 39 is alkylated at thephenolic oxygen using an alkylene glycol halide, forming ether 40. Theester moiety of 40 is hydrolyzed, providing acid 41, the carbonyl moietyof which is reduced to CH₂, affording 42. The hydroxyl moiety ofcompound 42 is oxidized to the corresponding aldehydes by the action ofNaOCl, providing 43. Trimer 44 is prepared by forming a Schiff base,which is subsequently reduced to the corresponding amine. Removal of theBoc protecting groups yields the amine scaffold 45.

Those of skill in the art will appreciate that the route set forth abovecan be practiced with substantially any substrate, including, but notlimited to amino acids with side chains (e.g., cysteine, glutamic acid,lysine, serine, etc.), as well as other non-amino acid species.

Reactive Functional Groups

As discussed above, the complexes of the invention are tethered to otherspecies by means of bonds formed between a reactive functional group onthe ligand or a linker attached to the ligand, and a reactive functionalgroup of complementary reactivity on the other species. For clarity ofillustration the succeeding discussion focuses on the conjugation ofrepresentative ligands and complexes of the invention to polymers,including poly(ethers) and dendrimers, and to targeting agents for bloodpool imaging. The focus exemplifies selected embodiments of theinvention from which, others are readily inferred by one of skill in theart. No limitation of the invention is implied, by focusing thediscussion on the representative embodiments.

Exemplary ligands and complexes of the invention bear a reactivefunctional group, which is generally located on the scaffold or on asubstituted or unsubstituted alkyl or heteroalkyl chain attached to thescaffold or a chelating moiety, allowing their facile attachment toanother species. A convenient location for the reactive group is theterminal position of the chain.

Reactive groups and classes of reactions useful in practicing thepresent invention are generally those that are well known in the art ofbioconjugate chemistry. Currently favored classes of reactions availablewith reactive ligand analogues are those proceeding under relativelymild conditions. These include, but are not limited to nucleophilicsubstitutions (e.g., reactions of amines and alcohols with acyl halides,active esters), electrophilic substitutions (e.g., enamine reactions)and additions to carbon-carbon and carbon-heteroatom multiple bonds(e.g., Michael reaction, Diels-Alder addition). These and other usefulreactions are discussed in, for example, March, ADVANCED ORGANICCHEMISTRY, 3rd Ed., John Wiley & Sons, New York, 1985; Hermanson,BIOCONJUGATE TECHNIQUES, Academic Press, San Diego, 1996; and Feeney etal., MODIFICATION OF PROTEINS; Advances in Chemistry Series, Vol. 198,American Chemical Society, Washington, D.C., 1982.

Exemplary reaction types include the reaction of carboxyl groups andvarious derivatives thereof including, but not limited to,N-hydroxysuccinimide esters, N— hydroxybenzotriazole esters, acidhalides, acyl imidazoles, thioesters, p-nitrophenyl esters, alkyl,alkenyl, alkynyl and aromatic esters. Hydroxyl groups can be convertedto esters, ethers, aldehydes, etc. Haloalkyl groups are converted to newspecies by reaction with, for example, an amine, a carboxylate anion,thiol anion, carbanion, or an alkoxide ion. Dienophile (e.g., maleimide)groups participate in Diels-Alder. Aldehyde or ketone groups can beconverted to imines, hydrazones, semicarbazones or oximes, or via suchmechanisms as Grignard addition or alkyllithium addition. Sulfonylhalides react readily with amines, for example, to form sulfonamides.Amine or sulfhydryl groups are, for example, acylated, alkylated oroxidized. Alkenes, can be converted to an array of new species usingcycloadditions, acylation, Michael addition, etc. Epoxides react readilywith amines and hydroxyl compounds.

Exemplary combinations of reactive functional groups found on a ligandof the invention and on a targeting moiety (or polymer or linker) areset forth in Table 2.

TABLE 2 Chemical Chemical Functionality 1 Functionality 2 LinkageHydroxy Carboxy Ester Hydroxy Carbonate Amine Carbamate SO₃ Sulfate PO₃Phosphate Carboxy Acyloxyalkyl Ketone Ketal Aldehyde Acetal HydroxyAnhydride Mercapto Mercapto Disulfide Carboxy Acyloxyalkyl ThioetherCarboxy Thioester Carboxy Amino amide Mercapto Thioester CarboxyAcyloxyalkyl ester Carboxy Acyloxyalkyl amide Amino Acyloxyalkoxycarbonyl Carboxy Anhydride Carboxy N-acylamide Hydroxy Ester HydroxyHydroxymethyl ketone ester Hydroxy Alkoxycarbonyl oxyalkyl Amino CarboxyAcyloxyalkylamine Carboxy Acyloxyalkylamide Amino Urea Carboxy AmideCarboxy Acyloxyalkoxycarbonyl Amide N-Mannich base Carboxy Acyloxyalkylcarbamate Phosphate Hydroxy Phosphate oxygen ester Amine PhosphoramidateMercapto Thiophosphate ester Ketone Carboxy Enol ester SulfonamideCarboxy Acyloxyalkyl sulfonamide Ester N-sulfonyl-imidate

One skilled in the art will readily appreciate that many of theselinkages may be produced in a variety of ways and using a variety ofconditions. For the preparation of esters, see, e.g., March supra at1157; for thioesters, see, March, supra at 362-363, 491, 720-722, 829,941, and 1172; for carbonates, see, March, supra at 346-347; forcarbamates, see, March, supra at 1156-57; for amides, see, March supraat 1152; for ureas and thioureas, see, March supra at 1174; for acetalsand ketals, see, Greene et al. supra 178-210 and March supra at 1146;for acyloxyalkyl derivatives, see, PRODRUGS: TOPICAL AND OCULAR DRUGDELIVERY, K. B. Sloan, ed., Marcel Dekker, Inc., New York, 1992; forenol esters, see, March supra at 1160; for N-sulfonylimidates, see,Bundgaard et al., J. Med. Chem., 31:2066 (1988); for anhydrides, see,March supra at 355-56, 636-37, 990-91, and 11 54; for N— acylamides,see, March supra at 379; for N-Mannich bases, see, March supra at800-02, and 828; for hydroxymethyl ketone esters, see, Petracek et al.Annals NYAcad. Sci., 507:353-54 (1987); for disulfides, see, March supraat 1160; and for phosphonate esters and phosphonamidates.

The reactive functional groups can be chosen such that they do notparticipate in, or interfere with, the reactions necessary to assemblethe reactive ligand analogue. Alternatively, a reactive functional groupcan be protected from participating in the reaction by the presence of aprotecting group. Those of skill in the art will understand how toprotect a particular functional group from interfering with a chosen setof reaction conditions. For examples of useful protecting groups, seeGreene et al., PROTECTIVE GROUPS IN ORGANIC SYNTHESIS, John Wiley &Sons, New York, 1991.

Generally, prior to forming the linkage between the ligand and thetargeting (or other) agent, and optionally, the linker group, at leastone of the chemical functionalities will be activated. One skilled inthe art will appreciate that a variety of chemical functionalities,including hydroxy, amino, and carboxy groups, can be activated using avariety of standard methods and conditions. For example, a hydroxylgroup of the ligand (or targeting agent) can be activated throughtreatment with phosgene to form the corresponding chloroformate, orp-nitrophenylchloroformate to form the corresponding carbonate.

In an exemplary embodiment, the invention makes use of a targeting agentthat includes a carboxyl functionality. Carboxyl groups may be activatedby, for example, conversion to the corresponding acyl halide or activeester. This reaction may be performed under a variety of conditions asillustrated in March, supra pp. 388-89. In an exemplary embodiment, theacyl halide is prepared through the reaction of the carboxyl-containinggroup with oxalyl chloride. The activated agent is combined with aligand or ligand-linker arm combination to form a conjugate of theinvention. Those of skill in the art will appreciate that the use ofcarboxyl-containing targeting agents is merely illustrative, and thatagents having many other functional groups can be conjugated to theligands of the invention.

Macromolecular Complexes

In an exemplary embodiment, the invention provides a Gd(III) complex ofthe invention that is macromolecular, i.e., MW>1000 D. In oneembodiment, a macromolecular complex of the invention is formed bycovalently conjugating a complex to a macromolecule via a reactivefunctional group. In another embodiment, the macromolecular complex isformed by a non-covalent interaction between a complex and amacromolecule, e.g, a serum protein.

In the following discussion, the invention is described by reference topoly(ethylene glycol) adducts of homopodal and heteropodal ligands andchelates. Those of skill in the art will appreciate that the focus onpoly(ethylene glycol) adducts is for clarity of illustration and doesnot limit the scope of the invention. Thus, the invention providesmacromolecular complexes that include components derived frombiomolecules and synthetic molecules. Exemplary biomolecules includepolypeptides (e.g., antibodies, enzymes, receptors, antigens);polysaccharides (e.g., starches, inulin, dextran); lectins, non-peptideantigens and the like. Exemplary synthetic polymers include poly(acrylicacid), poly(lysine), poly(glutamic acid), poly(ethylene imine), etc.

Covalent Conjugation

Selection of an appropriate reactive functional group on a complex ofthe invention to form a desired macromolecular species is well withinthe abilities of one of skill in the art. Exemplary reactive functionalgroups of use in forming the covalent conjugates of the invention arediscussed above. It is well within the abilities of one of skill toselect and prepare a ligand of the invention having an appropriatereactive functional group of complementary reactivity to a reactivegroup on its conjugation partner.

In one embodiment, the bond formed between reactive functional groups ofthe macromolecule and that of the complex attaches the ligand to themacromolecule essentially irreversibly via a “stable bond” between thecomponents. A “stable bond”, as used herein, is a bond, which maintainsits chemical integrity over a wide range of conditions (e.g., amide,carbamate, carbon-carbon, ether, etc.). In another embodiment, themacromolecule and the complex are linked by a “cleaveable bond”. A“cleaveable bond”, as used herein, is a bond that undergoes scissionunder selected conditions. Cleaveable bonds include, but are not limitedto, disulfide, imine, carbonate and ester bonds. As discussed in thepreceding sections, the reactive functional group can be located at oneor more positions of the scaffold and/or one or more positions on thechelating ring structures.

Macromolecular Conjugates

The present invention also provides conjugates between the chelatingagents and complexes of the invention and linear, branched and cyclicpolymers, e.g., polysaccharides, poly(amino acids), cyclodextrins,synthetic polymers, etc.

Macromolecular Complexes with Increased Relaxivity

A promising route to the optimization of proton relaxivity involvesincreasing the rotational correlation time, τ_(r), by increasing themolecular weight of the complex, resulting in slower rotation of themolecule in solution. This has been achieved in a number of ways.Several groups have investigated polymer-based contrast agents in whicha DTPA chelate is attached to a polymer such as polylysine,poly(ethylene glycol) or polydextran (Dresser, T. R. et al., J. Magn.Reson. Imaging 1994, 4, 467; Toth, E. H. et al., Chem. Eur. J. 1999, 5,1202; Corot, C. S. et al., D. Acta Radiologica 1997, 38, 91). Improved(but modest) relaxivities are typically observed for these polymericagents, reflecting the fast local motions within the polymer chain andthe slow water exchange due to the poly(amino-carboxylate) chelate used.Such systems are being investigated for blood pool imaging, particularlyas intravascular MRI contrast agents for myocardial perfusion (Casali,C. J. et al., Acad. Radiol. 1998, 5, S214).

Polysaccharides

The present invention provides conjugates between oxygen donor ligandsand saccharides, e.g., polysaccharides. In an exemplary embodiment, theinvention provides a conjugate between an oxygen donor chelate andinulin. Inulin is a naturally occurring polysaccharide which has beenpreviously investigated as a carrier for Gd(III) chelates (using DTPAand DOTA derivatives) (Rongved, P. K., J. Carbohydr. Res. 1991, 214,315; Corsi, D. M. V. E. et al., Chem. Eur. J. 2001, 7, 64). Theattachment of 37 [Gd(DO3ASQ)] (SQ=squarate) units to a 25 chain lengthinulin polymer was recently reported (Corsi, D. M. V. E. et al., Chem.Eur. J. 2001, 7, 64). This resulted in a ca. nine-fold increase in τ_(r)(735 ps) which resulted in a relaxivity of 20.3 mM⁻¹ s⁻¹ (at 20 MHz and37° C.). As with most derivatives of DTPA and DOTA, higher relaxivitywas limited by a very slow τ_(m) (260 ns in this case).

The structure of inulin can be described as a mixture of linearβ-(2→1)-linked α-D-fructofuranosyl chains with a α-D-glucopyranosyl unitat the terminal end. Inulin is commercially available in a variety ofmolecular weights and the degree of polymerization varies from 10 to 30,resulting in a molecular weight distribution of 1500 to 5000 Da. Thehigh hydrophilicity, pH stability, low solution viscosity andbiocompatability of inulin should ensure that the conjugated-MRI agenthas favorable pharmacological properties.

In an exemplary embodiment, the inulin is attached to a TREN-HOPOscaffold. The bond is formed using an activated intermediate of acarboxylic acid derivative that is attached to the endocyclic nitrogenof a HOPY moiety. Other modes of conjugation are well within the skillof those of skill in the art. For example, similar molecules are readilysynthesized using HOPO-TAM moieties in which the amide moiety of the TAMmoiety is activated, and subsequently conjugated to inulin.

An exemplary synthetic scheme to attach TREN-6-Me-HOPO to the backboneof inulin is shown in Scheme 8. The flexibility of the polymer chainallows for the preparation of derivatives that have high degrees ofsubstitution, facilitating a higher rigidity in the Gd(III) complex.

Also included in the present invention is a method of preparing achelating agent having a polymeric backbone and at least onefunctionality to which a chelating ligand of the invention is bonded.Examples of suitable polymers include, but are not limited to,poly(styrene-divinylbenzene), agarose (manufactured by Bio-Rad Corp.,Richmond, Calif., under the name “Affi-Gel”), and polyacrylamide. Thoseof skill in the art will appreciate that the method of the invention isnot limited by the identity of the backbone species, and that numerousamine-, hydroxyl- and sulfhydryl-containing compounds are useful asbackbones in practicing the method of the invention.

Dendrimer-Based Agents

In another aspect, the present invention provides a metal chelate as setforth above, which is attached to a dendrimer via a reactive functionalgroup. Similar to the polymeric group discussed above, the dendrimer hasat least two reactive functional groups. In one embodiment, one or morefully assembled ligand is attached to the dendrimer. Alternatively, thethe chelate is formed directly on the dendrimer.

In contrast to linear polymers, dendrimers have a relatively rigidstructure and the overall tumbling of the molecule contributessignificantly to the rotational correlation time of the Gd—H vector. Thehigh monodispersity, minimal shape variation and uniform surfacechemistry of Gd(III) chelate dendrimers are also key features importantin their potential application as MRI agents (Krause, W. et al., Top.Curr. Chem. 2000, 210, 261). Although the rotational correlation timeincreases with higher generation dendrimers, the increase in protonrelaxivity eventually reaches a plateau, an effect which is attributedto the slow water exchange (τ_(m)) inherent to the Gd(III) DTPA- andDOTA-based systems (Toth, E. et al., Chem. Eur. J. 1996, 2, 1607).Hence, the reported relaxivity values for dendrimer conjugates rangefrom 11 to 36 mM⁻¹s⁻¹ per Gd(III) center, depending on the nature of thechelate and the dendrimer structure (Dong, Q. H. et al., Invest. Radiol.1998, 33, 699; Wiener, E. C. B. et al., Magn. Reson. Med. 1994, 31, 1;Adam, G. et al., Magn. Reson. Imaging 1994, 4, 462; Margerum, L. D. etal., J. Alloys Compd. 1997, 249, 185; Bryant, L. H. et al., InProceedings of the 6th International Society of Magnetic Resonance inMedicine Conference: Sydney, Australia, 1998).

Gd(III)-chelate-dendrimer conjugates have enhanced relaxivity due totheir spherical rigid structure (which results in a slower τ_(r)).According to the Enhanced Permeation and Retention Effect, highgeneration dendrimers of high molecular weight should be preferentiallyuptaken by tumor cells (Narayanan, V. V. et al., Macromolecules 2000,33, 3944; Wiener, E. C. et al., J. Am. Chem. Soc. 1996, 118, 7774;Maeda, H. et al., J. Controlled Release 2000, 75, 271; Malik, N. et al.,J. Controlled Release 2000, 65, 133). Previous work has shown, however,that the solubility of the macromolecular complex depends both on theproperties of the Gd chelate and on the dendrimer itself (Cohen, S. M.et al., Chem. Eur. J. 2000, 6, 2). The highly water-soluble PAMAMdendrimers have yielded insoluble compounds upon functionalization withhydrophobic 1-Me-HOPO.

In an exemplary embodiment, a water-soluble and bio-adapted polyester(polypropionate) class of dendrimer contrast agents has been designed toprovide favorable pharmacokinetic properties. See, for example, Ihre, H.et al., Macromolecules 1998, 31, 4061; Ihre, H. et al., J. Am. Chem.Soc. 1996, 118, 6388; Anders, H., Ihre, H., Patent W0/9900440 (Sweden)).In an exemplary embodiment, the termini of the dendrimers are conjugatedto an oxygen donor ligand of the invention, e.g., TREN-1-Me-HOPO-TAM.The dendrimers are readily functionalized with thiaz-activated oxygendonor ligands, such as activated TREN-1-Me-HOPO-TAM ligands beforecomplexation with Gd³⁺ (Dong, Q. H. et al., Invest. Radiol. 1998, 33,699).

Chelating Agents Containing PEG Functionalization

In another embodiment, the invention provides a complex, which includesa structure according to Formula I in which at least one of R¹, R², R³,and R⁴ comprise a moiety derived from poly(ethyleneglycol). In anexemplary embodiment, the invention provides derivatives ofTREN-1-Me-3,2-HOPO-TAM, such as Gd-TREN-bis-(1-Me-HOPO)-(TAM-PEG-2000)(46); and Gd-TREN-bis-(1-Me-HOPO)-(TAM-PEG-5000) (47). A complex havinga PEG of average molecular weight 450 was also prepared.

In the exemplary compounds above, PEG moieties of average molecularweights 2000 and 5000, respectively, were attached to the amide of theTAM moiety of a TREN-bis-(1-Me-HOPO)-(TAM). Both complexes were of highsolubility in H₂O, allowing the water residence lifetime (τ_(m)) to bedetermined by a variable temperature 170 NMR study of the transverserelaxation rate of H₂ ¹⁷O. Significantly, there is an increase in τ_(m)as the PEG chain is lengthened, with values of approximately 8±1, 10±1,19±2 and 31±2 ns for TREN-1-Me-3,2-HOPO-TAM,Gd-TREN-bis-(1-Me-HOPO)-(TAM-PEG-450) (48),Gd-TREN-bis-(1-Me-HOPO)-(TAM-PEG-2000) (46),Gd-TREN-bis-(1-Me-HOPO)-(TAM-PEG-5000) (47), respectively (FIG. 9).These values span a range that is considered optimal for achievingmaximum values of proton relaxivity. The optimal value for τ_(m) dependson several variables, especially the field strength of the MRI scannerinstrument. Thus the incorporation of PEG chains into HOPO-TAM ligandsprovides a method for the optimization of a MRI contrast agent to aparticular field strength.

In addition to systematically altering the τ_(m), the method of theinvention provides for the optimization of additional parametersrelevant to a MRI contrast agent. For example, the relaxivity of thecore complexes is enhanced by their substitution with PEG (FIG. 10).

As will be apparent to those of skill in the art, the present inventionprovides compounds in which the PEG moiety is conjugated to positionsother than the amide of a TAM group of a TREN-bis-(1-Me-HOPO)-(TAM). Forexample, the PEG moiety may be tethered to the endocyclic nitrogen oranother position of the pyridinone moiety, another chelating moiety, thescaffold, or a combination of these positions.

Polyethylene glycol (PEG) is used in biotechnology and biomedicalapplications. The use of this agent has been reviewed (POLY(ETHYLENEGLYCOL) CHEMISTRY: BIOTECHNICAL AND BIOMEDICAL APPLICATIONS, J. M.Harris, Ed., Plenum Press, New York, 1992). Modification of enzymes(Chiu et al., J. Bioconjugate Chem., 4: 290-295 (1993)), RGD peptides(Braatz et al., Bioconjugate Chem., 4: 262-267 (1993)), liposomes(Zalipsky, S. Bioconjugate Chem., 4: 296-299 (1993)), and CD4-IgGglycoprotein (Chamow et al., Bioconjugate Chem., 4: 133-140 (1993)) aresome of the recent advances in the use of polyethylene glycol. Surfacestreated with PEG have been shown to resist protein deposition and haveimproved resistance to thrombogenicity when coated on blood contactingbiomaterials (Merrill, “Poly(ethylene oxide) and Blood Contact: AChronicle of One Laboratory,” in POLY(ETHYLENE GLYCOL) CHEMISTRY:BIOTECHNICAL AND BIOMEDICAL APPLICATIONS, Harris, Ed., Plenum Press, NewYork, (1992), pp. 199-220).

Many routes are available for attaching a chelate of the invention ontoa polymeric or oligomeric species. See, for example, Dunn, R. L., etal., Eds. POLYMERIC DRUGS AND DRUG DELIVERY SYSTEMS, ACS SymposiumSeries Vol. 469, American Chemical Society, Washington, D.C. 1991;Herren et al., J. Colloid and Interfacial Science 115: 46-55 (1987);Nashabeh et al., J. Chromatography 559: 367-383 (1991); Balachandar etal., Langmuir 6: 1621-1627 (1990); and Burns et al., Biomaterials 19:423-440 (1998).

Many activated derivatives of poly(ethyleneglycol) are availablecommercially and in the literature. It is well within the abilities ofone of skill to choose, and synthesize if necessary, an appropriateactivated PEG derivative with which to prepare a substrate useful in thepresent invention. See, Abuchowski et al. Cancer Biochem. Biophys.,7:175-186 (1984); Abuchowski et al., J. Biol. Chem., 252: 3582-3586(1977); Jackson et al., Anal. Biochem., 165: 114-127 (1987); Koide etal., Biochem Biophys. Res. Commun., 111: 659-667 (1983)), tresylate(Nilsson et al., Methods Enzymol., 104: 56-69 (1984); Delgado et al.,Biotechnol. Appl. Biochem., 12: 119-128 (1990)); N-hydroxysuccinimidederived active esters (Buckmann et al., Makromol. Chem., 182: 1379-1384(1981); Joppich et al., Makromol. Chem., 180: 1381-1384 (1979);Abuchowski et al., Cancer Biochem. Biophys., 7: 175-186 (1984); Katreetal. Proc. Natl. Acad. Sci. U.S.A., 84: 1487-1491 (1987); Kitamura etal., Cancer Res., 51: 4310-4315 (1991); Boccu et al., Z. Naturforsch.,38C: 94-99 (1983), carbonates (Zalipsky et al., POLY(ETHYLENE GLYCOL)CHEMISTRY: BIOTECHNICAL AND BIOMEDICAL APPLICATIONS, Harris, Ed., PlenumPress, New York, 1992, pp. 347-370; Zalipsky et al., Biotechnol. Appl.Biochem., 15: 100-114 (1992); Veronese et al., Appl. Biochem. Biotech.,11: 141-152 (1985)), imidazolyl formates (Beauchamp et al., Anal.Biochem., 131: 25-33 (1983); Berger et al., Blood, 71: 1641-1647(1988)), 4-dithiopyridines (Woghiren et al., Bioconjugate Chem., 4:314-318 (1993)), isocyanates (Byun et al., ASAIO Journal, M649-M-653(1992)) and epoxides (U.S. Pat. No. 4,806,595, issued to Noishiki etal., (1989). Other linking groups include the urethane linkage betweenamino groups and activated PEG. See, Veronese, et al., Appl. Biochem.Biotechnol., 11: 141-152 (1985).

The PEG group is preferred for two reasons. First, the highwater-solubility associated with PEG chains increases the rather lowsolubility of the parent complexes. Second, although it has beenpreviously found that rapid internal motions within a PEG chain resultin only a modest increase in τ_(r)(Toth, E. et al., Magn. Reson. Chem.1998, 36, S125), it has also been demonstrated that PEG chains can bindto HSA across a wide pH range (Azegami, S. T. et al., Langmuir 1999, 15,940). The value of relaxivity observed for the HSA complex isconsiderably higher than any other relaxivities (per Gd^(III) center)reported to date, reflecting an optimized water exchange rate and a slowrotational correlation time. Therefore, this non-covalent interaction toeffect an increase in τ_(r) and relaxivity has been exploited in thepresent invention.

Biospecific Contrast Agents

The development of MRI agents with higher tissue/organ specificity isseen as a large part of the future of magnetic resonance imaging(Caravan, P. E. et al., Chem. Rev. 1999, 99, 2293). Methods of enhancingthe specificity of agents include, but are not limited to, attaching thesignal generating portion of the agent (e.g., an oxygen donor ligandcomplex with Gd(III) to a species that actively directs the conjugate toa selected tissue. Exemplary active agents are antibodies and ligandsfor biologically relevant receptors. Another approach to improving thebiodistribution of an agent relies on preparing an agent that interactspassively (non-specifically) with a biologically relevant species, e.g,a serum protein. Yet another strategy exploits the enhanced permeabilityand uptake mechanism in which a damaged or diseased tissuepreferentially uptakes a macromolecular agent.

In an exemplary embodiment, the present invention provides an agent thatbinds to serum proteins in vivo, thus, affording a blood pool contrastenhancing agent. A second advantage accrues from the interaction betweenthe oxygen donor-metal complexes of the invention and a serum protein;the rotational correlation time of the complex increases, leading to anincrease in the relaxivity of the complex (Lauffer, R. B., Magn. Reson.Med. 1991, 22, 339).

In vivo binding to a macromolecule allows the Gd(III) complex to take ona rotational correlation time that is similar to that of themacromolecule, leading to a dramatic increase in relaxivity.Additionally, the binding causes an increased concentration andretention of the Gd(III) complex in the localized region of thebiomolecule. Furthermore, the relaxivity of the bound complex is muchgreater than that of the unbound complex, which leads to a hightarget-to-background ratio.

The complex MS-325 forms a noncovalent adduct with the blood proteinhuman serum albumin (HSA) (Parmalee, D. J. W. et al., Invest. Radiol.1997, 32, 741; Lauffer, R. B. P. et al., Radiology 1998, 207, 529). Therelaxivity of the resulting HSA adduct is 42.0 mM⁻¹s⁻¹, which is nearlyseven times greater than the in vitro relaxivity of the free complex inwater (6.6 mM⁻¹s⁻¹). MS-325 is currently in phase II and phase IIIclinical trials for imaging the cardiovascular system. Complexes havealso been designed to target other macromolecules. For example, Gd-BOPTAwas designed to target hepatocytes in order to facilitate hepatobiliaryimaging (Cavanga, F. M. et al., Invest. Radiol. 1997, 32, 780). This MRIcontrast agent has a relaxivity of 4.4 mM⁻¹s⁻¹ in water, 6.9 mM⁻¹s⁻¹ inrat plasma and 30 mM⁻¹s⁻¹ in rat hepatocytes.

Despite the success of second-generation contrast agents such as MS-325and Gd-BOPTA, there still remains a considerable difference betweenrelaxivities that have been achieved and the maximum relaxivities thatare theoretically possible. The primary reason for this is the slowwater exchange rates at the Gd(III) center. Optimal water residencetimes (τ_(m)) have been estimated to be a few tens of nanoseconds (Aime,S. B. et al., Chem. Soc. Rev. 1998, 27, 19), with the exact valuedepending on several variables, including the magnetic field strength ofthe MRI scanner. The water residence lifetimes of commercial contrastagents are typically 100-1000 ns, which is far slower than optimal.Although this will not limit the relaxivity of small, fast-tumblingmolecules to a great extent, the effect becomes much more significantfor molecules of long rotational correlation time (Toth, E. H. et al.,Chem. Eur. J. 1999, 5, 1202). All macromolecular contrast agents in useor under development incorporate the same basic DTPA, DTPA-BMA or DOTAchelate, resulting in slow water exchange and relatively modest protonrelaxivities.

The present invention provides slowly tumbling Gd(III) complexes withfaster water exchange rates (i.e. with τ_(m) and τ_(r) preferably in thelow ns regime). The invention also provides MRI contrast agents withdesirable in vivo persistence, which facilitate diagnosis ofphysiological abnormalities in specific regions of the body over longerperiods than currently possible. In the process of optimizingbiomolecule affinity, τ_(r), and τ_(m), other essential characteristics(such as stability and water-solubility) have not be worsened.Therefore, the new MRI contrast agents, which are the subject of thisinvention, contain the requisite structural features that result inimages with better morphological and functional information (Comblin, V.et al., Coord. Chem. Rev. 1999, 185-186, 451).

In an exemplary embodiment, the complexes of the invention include thecovalent attachment of substituents to the oxygen donor ligand core,which facilitate non-covalent interactions with endogenous biomolecules.Along with greater specificity in diagnosis, the binding of a slowlyrotating macromolecule to the contrast agent allows for longerrotational correlation times in vivo, which could dramatically increasethe relaxivity.

Blood Pool MRI Contrast Agents

The present invention is exemplified by reference to blood pool contrastagents. The residence of the complexes of the invention in the bloodpool is influenced by a number of structural features. For example, themolecular size (weight) of the complexes of the invention is readilyincreased to a value that prevents their rapid elimination by glomerularfiltration. The increase in molecule size can be achieved by attaching amacromolecular moiety to the ligand. Alternatively, an array of ligandsmay be attached to a macromolecular carrier, such as a poly(peptide),poly(saccharide), or a dendrimer or other synthetic polymer. In generalit is preferred that the resulting construct is biocompatible,substantially non-immunogenic, water-soluble and has an acceptablerelaxivity. When the ligands (or complexes) of the invention areattached to a macromolecular species, the attachment can be through astable linkage or a linkage that is cleaved under biologically relevantconditions. Cleaveable linkages include a cleaveable group as discussedherein. Strategies for preparing biodegradable contrast agents are knownin the art. See, for example, U.S. Pat. No. 6,312,664.

Another strategy for enhancing the blood pool residence time of achelate of the invention relies on the non-covalent, reversibleinteraction between the chelate and a component of the blood, forexample, a protein.

Human serum albumin (HSA) is an attractive target for blood pool imagingas it constitutes 4.5% of plasma and is the most abundant protein inserum. Binding of the MRI contrast agent to HSA serves three purposes.First, it targets the complex to the blood pool allowing selectiveenhancement of arteries and veins during MR angiographic evaluations.Second, protein binding slows down the molecular tumbling time of thecomplex and should provide a 5- to 10-fold increase in relaxationenhancement as compared to the parent complex (Caravan, P. et al.,Inorg. Chem. 2001, 40, 2170). Finally, albumin binding increases thehalf-life of the drug in vivo, which allows the radiologist time toimage multiple body regions and to employ pulse sequences which givehigh resolution images. However, HSA possesses numerous binding sites ofdifferent levels of hydrophobicity and hydrophilicity, hence, differentaffinities for small molecules (FIG. 11). The warfarin binding site (insubdomain IIA of HSA) is one such cavity and a recent study (Zaton, A.M. L. V., J. P. Chem.-Bio. Int. 2000, 124, 1) has shown that4-hydroxycoumarin (CMN) has a very high affinity for this site incomparison with structural analogues and uracils (pyrimidinederivatives). The synthetic scheme the attachment of CMN toTREN-6-Me-HOPO is shown in Scheme 9.

In Scheme 9, the protected HOPO derivative is combined withhydroxy-coumarin in the presence of formaldehyde to form the protectedcoumarin-HOPO adduct. The adduct is subsequently reacted with an aminebackbone as discussed herein and, optionally, with another chelatingmoiety, such as a HOPO substituted on the endocyclic nitrogen with anacetic acid residue. The benzyl protecting groups are optionally removedafter the synthesis of the ligand is complete.

Other exemplary HSA-binding ligands of the invention include ahydrophilic moiety and a hydrophobic group, e.g., a benzyl group (FIG.12). In one embodiment, the hydrophilic group is PEG. In anotherrepresentative embodiment, the hydrophilic group is a species such as acarboxylic acid. Presently preferred hydrophobic moieties includesubstituted or unsubstituted benzyl groups. Representative ligandsaccording to this motif include:

It has been found that the affinity of the complex for HSA is modulatedby varying the structure of the PEG substituent (FIG. 13). Complexbinding to HSA enhances the relaxivity of the complex (FIG. 14). Thehydrophobic and hydrophilic moieties may be attached to the same ordifferent chelating subunits (FIG. 15).

Attachment of benzyl (Bn) or para-methoxy-benzyl (MOB) groups to thepyridine nitrogen of the 6-Me-HOPO scaffold is readily accomplishedaccording to Scheme 10.

In Scheme 10, the ethyl ester (10) is hydrolyzed to the correspondingacid 49, which is converted to thiazolide 50. Two thiazolide units arecoupled to the TREN backbone, affording a ligand having a free aminegroup 51 to which is coupled a protected PEG-TAM derivative 52. Theresulting heteropodal ligand 53 is debenzylated, liberating the phenolichydroxl moieties of the TAM subunit. The resulting compound 54 ismetallated to provide Gd(III) chelate 55.

As shown in Scheme 11, the method of Scheme 10 is equally applicable tothe preparation of oxygen donor ligands in which the benzyl group issubstituted.

Scheme 12 sets forth the synthesis of thiaz-TAM-(L)Phen-TRI, which isused to form oxygen donor ligands of the invention. Thus, N-protectedphenylalanine 59 is activated as the thiazolide 60, which is coupled toan amino-PEG, affording PEGylated phenylalanine 61. The BOC protectinggroup is then cleaved yielding the amine 62. Compound 62 is coupled todi-thiaz-TAM 23, to provide compound 63, having both hydrophobic andhydrophilic characteristics. Compound 63 is coupled to a ligand scaffold(such as 24) as discussed above.

The Gd complexes of the invention interact with HSA. It is within thescope of the invention to vary the substituents on the oxygen donorligand to adjust the water solubility of the complex. Furthermore, thelinker between the protein binding moiety and the Gd chelate can beadjusted to tune the degree of protein-ligand interaction.

Liver-Specific MRI Agents

The invention also provides complexes that are targeted to the liverand/or other components of the reticuloendothelial system. In anexemplary embodiment, the complex includes a hydrophobic moiety or othergroup known to be preferentially taken up by the liver or othercomponents of the reticuloendothelial system. For example, a directroute to liver-specific MRI agents is to take advantage of the knownbiodistribution of cholesterol. Cholesterol is a biomolecule that issynthesized in the liver and metabolized in the liver and bile glands.

In an exemplary embodiment a cholesterol derivative, e.g., cholic acid(CHOL) is attached to the TREN-HOPO-TAM scaffold (Scheme 13). Thisattachment enables specific in vivo localization of the conjugated-MRIagent to tissue of the reticuloendothelial system (Anelli, P. L. et al.,Bioconj. Chem. 1999, 10, 137; Anelli, P. L. et al., Acta Radiologica1997, 38, 15) resulting in hepatobiliary contrast enhancement. Thenon-covalent binding of [Gd(TREN-HOPOAC-TAM-CHOL)(H₂O)₂]⁻¹ tohepatocytes also results in slower tumbling of the molecule resulting ina higher proton relaxivity. Cholic acid is non-toxic and is not expectedto be cleaved from the MRI agent while in the liver (Anelli, P. L. etal., Bioconj. Chem. 1999, 10, 137; Anelli, P. L. et al., ActaRadiologica 1997, 38, 15).

In Scheme 13, cholic acid is activated and converted to the ethylaminederivative 64 by treating the activated intermediate withethylenediamine. Intermediate 64 is combined with an active thiazolidederivative of a ligand of the invention 65, to form the correspondingcholesterol derivatized ligand 66.

In another exemplary embodiment, the invention provides liver-selectivecomplexes that include both a poly(ether) and a hydrophobic moiety, suchas a cholesterol derivative. A representative synthetic route isprovided in Scheme 14.

In Scheme 14 a ligand functionalized with a poly(ether) terminating withan active amine 67 is combined with activated cholesterol thiazolide 68to provide the bifunctionalized ligand, bearing both a poly(ether) and acholesterol moiety 69. Following the formation of the bifunctionalizeligand, the benzyl protecting groups are removed from the phenolicoxygens of the TAM moiety, affording ligand 70.

Tumor Selective Agents

The present invention also provides agents that are selective fortumors. In an exemplary embodiment, the invention provides a gadoliniumchelate conjugated to a sapphyrin. Sapphyrins, which are expandedporphyrins, have been studied extensively for use in medicinalapplications as anion binders and photosensitizers and have also beenstudied in cancer models to target tumors of pancreatic adenocarcinoma(constituting more than 75% of all pancreatic cancer). Therefore, thecombination of pancreas specificity of porphyrins, (cheap and readilyavailable for screening) and expanded porphyrins, and the fast waterexchange rate of the chelates of the invention can be combined to givepromising pancreas-selective and fast water exchangingGd-TREN-HOPO-porphyrin agents.

Pancreatic cancer most often occurs in the form of pancreaticadenocarcinoma. Very recent and exciting research shows that awater-soluble derivative of sapphyrin accumulates specifically in humanpancreatic adenocarcinoma when transplanted into and grown in mice(Ferucci. J. T. Annals of Oncology 10, Suppl. 4: S18 (1999)). In thisstudy, thin cross-sections of tissue were analyzed by UV and resonanceRaman spectroscopy for the sapphyrin, which was found to be present inthe tumor in large quantities at all post-injection times. At themaximum tumor concentration, pancreatic tumor selectivity is hundreds oftimes greater than that for the liver, kidney or muscle tissues in themouse model.

An exemplary synthesis of the compounds of the invention is set forth inScheme 15.

A C₂ symmetric sapphyrin is disubstituted with amine pendants. Theaddition of sapphyrin derivative 71 (R=Me) and two equivalents of 23form a disubstituted derivative 72 which is then be treated with 2 eq.of 24 in known amine coupling to form 73, which preserves the integrityof the [TREN-HOPO-TAM] geometry. Ligand 73 is further deprotected andGdCl₃ is combined with 73 to form 74.

The present invention also provides water soluble sapphyrin-Gd(III)complexes. In an exemplary embodiment, the sapphyrin is derivatized witha polyethylene glycol moiety, to counter the lipophilicy of thesapphyrin. The PEG pendant of 75 can be tethered at thehydroxypyridinoate nitrogen as shown in Scheme 16. Chlorinated 76 isthen added directly to 12 at the HOPO ring nitrogen formingmonosubstituted 77. Derivative 77 can be further combined with 24 and beincorporated in the second step of Scheme 15.

In another exemplary embodiment, the TREN is functionalized with PEG ina manner that is not detrimental to the stability of the complex. Thus,one or more of the three TREN arms is functionalized by Tf-protection ofthe amine nitrogen with the concomitant labilizing of the vicinal protonfor substitution by sapphyrin with alcohol pendant arms (Scheme 17). Anexemplary complex has a Gd to sapphyrin ratio of 4:1. In Scheme 17, TREN(commercially available) is protected, mono-iodinated and combined with80. The resulting product is reduced to form 81. This tetra-TREN can beflexibly used to prepare compounds of the invention.

In a further exemplary embodiment, the compounds of the invention arefurther conjugated to quinacrine or secretin to further enhance pancreasaffinity due to their individual molecular recognition, and pancreatictumor selectivity. Firstly, quinacrine, 82, has an affinity for acidicphospholipids and the enzyme pancreatic phospholipase A2. The acridinegroup also serves as a fluorescent marker, providing a means ofdetecting the compound in tissues. Compound 82 which is commerciallyavailable can be Suzuki coupled with a borane functionalizedpropeneamine (commercially available) and then undergo amide couplingwith 23 to yield the tethered quinacrine-TAM fragment (Scheme 18). 84 isutilized in place of 23 (Scheme 15, Step 1) in conjunction withtetra-TREN 81 in place of reagent 24 (0.25 eq.) to prepare a Gdmulticonjugate.

Secretin (commercially available) can also be conjugated at its Nterminus. Secretin, a 27-amino acid polypeptide, is received in vivo onits carboxylate end at pancreatic parenchymal cells. The N-terminus maybe selectively iodinated in reasonable yield prior to substitution atthe 6-Me-HOPO nitrogen.

Compound Characterization

The present invention generally utilizes art-recognized methods tocharacterize the new ligands and their metal complexes. The followingsections provide exemplary methods of characterizing the compounds ofthe invention. The methods set forth below are intended to illustrateuseful techniques for characterizing the compositions of the invention,but should not be construed as limiting the methods of us incharacterizing the compositions of the invention.

Methods of determining stability constants include, but are not limitedto those set forth in, Johnson, A. R. et al., Inorg. Chem. 2000, 39,2652-2660; and Cohen, S. M. X. et al., Inorg. Chem. 2000, 39, 5747.

The Bjerrum method can be used for metal complex stability measurements(pH titrations of ligand and metal+ligand). Competition titrations withDTPA can be performed to determine the stability of very stablecomplexes where direct pH titration methods are inappropriate.Spectrophotometric techniques can be used to monitor metal-ligandcomplexation reactions, which give rise to changes in the Vis/UV spectrarelative to the parent metal and ligand species. Withdigitally-recording automated spectrophotometeric titrators, factoranalysis of the Vis/UV spectra readily determined the species insolution, their individual spectra and the equilibrium constants whichinterrelate them.

Experimental Determination of Water Solubility

To attain neutral pH, a 0.01 M solution of buffer (HEPES, pKa 7.55) in0.1 M KCl was made in its acidic form and neutralized to pH 7 with KOH.A dilute solution (of the metal complex) of known concentration wasprepared and the UV-Vis spectrum acquired. The extinction coefficient(ε) at the maximum absorbance (at wavelength typically 320-360 nm) wasthen calculated using Beer-Lambert's law. A saturated solution of themetal complex was then prepared and the UV-Vis spectrum acquired and themaximum absorbance (at wavelength typically 320-360 nm) recorded. Incases where the saturated solution produced extremely high absorbances,the saturated solution was diluted and then the spectrum recorded. Usingthe known ε, the concentration of the saturated solution was thendetermined. This corresponds to the minimum solubility of the complex inH₂O (pH 7, 25° C., 0.1 M). The solubility of the complexes were observedto increase at lower salt concentration. The spectra of the complexes atpH 7 are consistent with the known ML spectra observed fromspectrophotometric solution thermodynamics studies of the metal-ligandcomplexation reactions and indicate the absence of free ligand (note:M=metal, L=ligand).

Typically, preliminary investigation of the minimum solubility of thecomplexes involves dissolving a known quantity of the complex in aprecise volume of solvent. The visible disappearance of particulatematter, over time or instantaneous in some cases, is indicative ofcomplete dissolution of the complex. Hence, a determination of theminimum solubility of the complexes is made before spectral studies areperformed.

Ligand Protonation Constants

The protonation constants for chelates of the invention are readilydetermined by potentiometric titrations. These constants are defined byeq 2:

$\begin{matrix}{{{H_{n - 1}L} + {\left. H^{+}\longleftrightarrow H_{n} \right.L\mspace{34mu} K_{n}}} = {\frac{\left\lbrack {H_{n}L} \right\rbrack}{\left\lbrack {H_{n - 1}L} \right\rbrack\left\lbrack H^{+} \right\rbrack}.}} & (2)\end{matrix}$Gd(III), Zn(II) and Ca(II) Formation Constants

Data for the spectrophotometric determination of formation constants forthe compounds of the invention (e.g., Tren-Me₂-5,4-HOPY) with gadoliniumare generally collected over a pH range of 2-8.

The formation constants are defined by eq 3:

$\begin{matrix}{{{mM} + {lL} + {\left. {hH}\longleftrightarrow M_{m} \right.L_{l}H_{h\mspace{20mu}}\mspace{20mu}\beta_{mlh}}} = \frac{\left\lbrack {M_{m}L_{l}H_{h}} \right\rbrack}{{{\left\lbrack M \right\}^{m}\lbrack L\rbrack}^{l}\lbrack H\rbrack}^{h}}} & (3)\end{matrix}$

The spectrophotometric data is refined using a model. For example, forGd[TrenHOPY], the model includes four components: LH₄ ⁺, GdL, GdLH⁺ andGdLH₂ ²⁺. This speciation is similar to the solution behavior ofTren-MOE-3,2-HOPO, although the parent complex, Gd(Tren-Me-3,2-HOPO),was not formerly reported as possessing a diprotonated species. (Johnsonet al., Inorg. Chem., 39:2652 (2000))

LogK_(ML) values reflect the metal-ligand affinity for deprotonatedligand in the reaction, M+L

ML. The pM value is one way to make allowance for the competition forthe ligand by protons in real solutions, and thus gives a more completepicture of the effectiveness of the ligand in chelating the metal.

The pGd values are defined by eq 4:pGd=−log[Gd] _(free) at pH 7.4 for [Gd]=1 μM, [L]=10 μM   (4)A pM value can be calculated for many conditions, but it is generallypreferable to calculate it for conditions relevant to biologicalconsiderations.

Potentiometric titrations of the ligand and/or complex with zinc andcalcium ions can also be performed. The potentiometric titrationsprovide a comparison to the high selectivity for Gd(III) afforded by theTrenHOPO system. (Xu et al., J. Am. Chem. Soc., 117:7245 (1995))

In an exemplary assay, solutions of a 1:1 ratio of metal ion to ligandare titrated over a pH range of 2.4-11. Generally, low concentrations(0.25 mM) are used to avoid precipitation in the pH region of 5.5-6.5for both the Ca²⁺ and Zn²⁺ systems.

Water Proton Relaxation

The efficacy of a paramagnetic complex as a possible MRI contrast agentdepends on its ability to catalyze the nuclear magnetic relaxation rateof solvent protons (Lauffer, R. B., Chem. Rev., 87, 901 (1987); Tweedle,M. F.; Kumar, K. In Magnetic Resonance Imaging (MRI) Contrast Agents;Clarke, M. I., Sadler, P. J., Eds.; Springer: Berlin, Germany, Vol. 2,p. 1 (1999); Koenig et al., Prog. NMR Spectrosc., 22:487 (1990)). Thisproperty, which is measured in terms in relaxivity, r_(1p), is definedas the enhancement of the water proton longitudinal relaxation rateinduced by a 1 mML⁻¹ solution of the paramagnetic compound at a giventemperature and magnetic field strength (Aime et al., Chem. Soc. Rev.,27:19 (1998)). In the absence of specific interactions, the values ofr_(1p) in vitro and in blood serum are quite comparable and so analysisof the relaxation properties of a paramagnetic solute in water are ofimportance for predicting the behavior in vivo.

¹H and ¹⁷O NMR Relaxivity Studies

The proton relaxivity of the new Gd(III) complexes are generallyassessed by art-recognized methods. See, for example, Aime, S. B. etal., Acc. Chem. Res. 1999, 32, 941; Aime, S. B. et al., Coord. Chem.Rev. 1999, 185-6, 321; Aime, S. B. et al., Chem. Soc. Rev. 1998, 27, 19;Aime, S. B. et al., Magn. Reson. Chem. 1998, 36, S200; Aime, S. B. etal., J. Biol. Inorg. Chem. 1997, 2, 470; Botta, M., Eur. J. Inorg Chem.2000, 399).

In an exemplary assay, the longitudinal water proton relaxation rate at20 MHz are measured (typically with a NMR spectrometer operating at 0.5T) with a reproducibility of the T₁ data to ±0.5%. Equipment for thecontrol of temperature (with an accuracy of ±0.1° C.) during thesemeasurements are employed.

The mean residence water lifetime, τ_(m), is generally determined bymeasuring the transverse ¹⁷O NMR relaxation rate (R_(2p)) at varioustemperatures. The variable-temperature ¹⁷O NMR measurement is performedusing spectrometers which operate at various magnetic field strengths(2.1 and 9.4 T are typically used) equipped with a 5 mm probe. A D₂Oexternal lock and solutions containing 2.6% of the ¹⁷O isotope are used.The observed transverse relaxation rates are calculated from the signalwidth at half-height. Details of the instrumentation, experimentalmethods, and data analysis are reported elsewhere and incorporated byreference herein (Cohen, S. M. X. et al., Inorg. Chem. 2000, 39, 5747;Aime, S. B. et al., J. Biol. Inorg. Chem. 1997, 2, 470; Aime, S. B. etal., Magn. Reson. Chem. 1998, 36, S200).

The Gd(III) complexes of the HOPO and HOPY systems set forth herein arepromising candidates for MRI contrast agents since they possess highthermodynamic stability and high relaxivity due to the presence of twocoordinated water molecules characterized by a fast rate of . Therelaxivity of the complexes of the invention, measured at 20 MHz, 25° C.and pH=7.2 is typically greater than 5.0 Mm⁻¹s⁻¹, with relaxivities offrom at least about 9.0 mM⁻¹s⁻¹ to about 11 mM⁻¹s⁻¹ being readilyobtained. The higher values are at least about two times higher thanthat of the currently used contrast agents based onpolyaminocarboxylate.

NMRD Studies

Measuring the relaxation rates of an abundant nucleus in a largemagnetic field range is called relaxometry. A relaxometry profile is aplot of nuclear magnetic relaxation rates, usually 1/T₁, as a functionof the Larmor frequency or the magnetic field on a logarithmic scale(see FIG. 7). This plot is also called a Nuclear Magnetic RelaxationDispersion (NMRD) curve. Whereas the measurement of relaxation rates isa routine task at higher magnetic fields (>1 MHz proton Larmorfrequency, 0.023 T), at lower fields the dramatic decrease insensitivity sets a practical limit. NMRD profiles of paramagneticsolutions show, however, very often interesting features at frequenciesbelow 1 MHz. This special experimental technique utilizes a fast cyclingof the magnetic field with a field-cycling relaxometer (Caravan, P. E.et al., Chem. Rev. 1999, 99, 2293; Toth, E. et al Top. Curr. Chem. 2002,221).

Nuclear magnetic resonance dispersion (NMRD) profiles can be used todetermine the values of the parameters that contribute to the relaxivityof a Gd(III) complex (e.g. q and τ_(r)) The methods involved measuringthe magnetic field strength (Larmor frequency) dependence of the solventproton longitudinal relaxation rate in the presence of a Gd(III)complex. The proton 1/T₁ NMRD profiles were typically measured on afield-cycling relaxometer over a continuum of magnetic field strengths.The relaxometer generally operates under computer control with anabsolute uncertainty in 1/T₁ of ±1%. Details of the instrumentation anddata acquisition procedure are reported elsewhere and are incorporatedby reference herein. (Aime, S. B. et al., J. Biol. Inorg. Chem. 1997, 2,470; Aime, S. B. et al., Magn. Reson. Chem. 1998, 36, S200; Cohen, S. M.X. et al., Inorg. Chem. 2000, 39, 5747).

Inner-Hydration Sphere Assessment

In an exemplary assay, the number of water molecules (q) bound to themetal ion is assessed by luminescence decay kinetics. In this method,the metal ions are directly excited with a powerful Nd-YAG laser and thedecay rates are recorded. In general, the method relies upon thepreparation of the terbium(III) complexes of the new ligands.Alternatively, the europopium (III) complexes of the new ligands aresynthesized.

Accurate assessment of q is obtained by comparing the decay rates of thecomplex in H₂O and D₂O (which minimally quenches emission) are comparedas the inner sphere water molecules quench the emission of the metalion.

Variable Temperature ¹⁷O NMR

The NMRD profiles of complexes of the invention are in the fast exchangeregime, a condition that precludes the assessment of the water exchangerate from the analysis of the magnetic field dependence of the protonrelaxivities. The value of τ_(M), a crucial parameter for the evaluationof the efficiency of a contrast agent, can be independently obtained bya variable temperature, proton decoupled ¹⁷O NMR measurement of thewater nuclear transverse relaxation rate (R₂) using a well-established(Powell et al., J. Am. Chem. Soc., 118:9333 (1996); Aime et al., Acc.Chem. Res., 32:941 (1999)) The R₂ values are dominated by the scalarrelaxation mechanism which depends on k_(ex) and its temperaturedependence (ΔH_(M)), the electronic relaxation rate and its temperaturedependence (Δ², τ_(V), ΔHv) and the hyperfine coupling constant A/h.

A standard value of −3.8×10⁶ rad s⁻¹ is used for the hyperfine couplingconstant and the values obtained by the analysis of the NMRD profilesfor electron relaxation (Δ² and τ_(V)). The data id fitted to theSwift-Connick equations.

Biomolecule Affinity Measurements

Biomolecule affinity measurements are generally measured byart-recognized methods. For example, the non-covalent interactionbetween HSA and the metal complexes containing hydrophobic groups havebeen thoroughly investigated using the well-established protonrelaxation enhancement (PRE) method (Caravan, P. E. et al., Chem. Rev.1999, 99, 2293) that allows both the binding parameters (K_(a)) and therelaxivity enhancement of the (Metal complex)-HSA adduct to bedetermined. In this method, the water proton relaxation rates ofsolutions containing the metal complex and increasing concentrations ofthe serum protein are measured.

An exemplary assay is performed with competitor probes (such as warfarinand ibuprofen), which elucidate the binding sites of the protein withwhich the MRI agents interact. The measurements are typically performedat pH 7.4 (in phosphate buffer) and 298 K, which are close tophysiological conditions.

In HSA, there are often multiple non-identical binding sites that maygive rise to varying degrees of proton relaxation enhancement. Hence, anindependent method for determining the binding constants may benecessary to complement the PRE method.

The emissive properties of Eu(III) and/or Tb(III), are used to reflectthe rates of emissive decay in distinct sites that the metal ionoccupies. Hence, luminescence titration of the Eu(III) and Tb(III)complexes of the ligands with HSA is a good method for determiningbiomolecule affinity (Feig, A. L. P. et al., Chem. & Biol. 1999, 6, 801;Chaudhuri, D. H. et al., Biochem. 1997, 36, 9674; Cronce, D. T. H. etal., Biochem. 1992, 31, 7963.

Biodistribution and Acute Toxicity Studies

The conjugation of metal chelates to substituents that target specificregions of the body necessitated biodistribution studies on the new MRIagents. Typically, a preliminary in vitro cell screen is performed inorder to identify the possible high affinity of these agents forspecific mammalian cells (e.g. myocytes and hepatocytes). The in vitrocell cytotoxicity of the agents is assessed using, for example, thetrypan blue exclusion method. Toxicity studies on healthy mice and ratsare performed to obtain the LD₅₀ data for the complexes.

In another exemplary assay, the in vivo biodistribution of the chelatingagents in tumor-induced rats is determined to assess tumor uptake of theagents. An exemplary assay involves radiotracer studies using ¹⁵³Gd. Therodent is induced with the tumor of choice and the tumor is allowed togrow over several weeks to a diameter suitable for biodistributionexperiments. The rodents are then transferred to a normal diet for anappropriate period of time. The radioactive MRI agent is then injectedand the urine and feces collected and analyzed for counts periodically.The animal is subsequently sacrificed and the organs removed forradioactive analyses. The biodistribution of the MRI agent as apercentage of the injected dose (% ID) per organ and as a %ID/g oftissue weight is then calculated. This is then compared with thedistribution of the MRI agent in a control group (healthy rodents of thesame cell line). A study on the in vivo biodistribution of the chelatingagents enables the identification of the localized distribution of theMRI agents in the presence and absence of tumors.

Pharmaceutical Formulations

The compounds of the present invention can be prepared and administeredin a wide variety of oral, parenteral and topical dosage forms. Thus,the compounds of the present invention can be administered by injection,that is, intravenously, intramuscularly, intracutaneously,subcutaneously, intraduodenally, or intraperitoneally. Also, thecompounds described herein can be administered by inhalation, forexample, intranasally. Additionally, the compounds of the presentinvention can be administered transdermally. Accordingly, the presentinvention also provides pharmaceutical compositions comprising apharmaceutically acceptable carrier or excipient and one or morecompounds of the invention.

For preparing pharmaceutical compositions from the compounds of thepresent invention, pharmaceutically acceptable carriers can be eithersolid or liquid. Solid form preparations include powders, tablets,pills, capsules, cachets, suppositories, and dispersible granules. Asolid carrier can be one or more substances, which may also act asdiluents, flavoring agents, binders, preservatives, tabletdisintegrating agents, or an encapsulating material.

In powders, the carrier is a finely divided solid, which is in a mixturewith the finely divided active component. In tablets, the activecomponent is mixed with the carrier having the necessary bindingproperties in suitable proportions and compacted in the shape and sizedesired.

The powders and tablets preferably contain from 5% or 10% to 70% of theactive compound. Suitable carriers are magnesium carbonate, magnesiumstearate, talc, sugar, lactose, pectin, dextrin, starch, gelatin,tragacanth, methylcellulose, sodium carboxymethylcellulose, a lowmelting wax, cocoa butter, and the like. The term “preparation” isintended to include the formulation of the active compound withencapsulating material as a carrier providing a capsule in which theactive component with or without other carriers, is surrounded by acarrier, which is thus in association with it. Similarly, cachets andlozenges are included. Tablets, powders, capsules, pills, cachets, andlozenges can be used as solid dosage forms suitable for oraladministration.

For preparing suppositories, a low melting wax, such as a mixture offatty acid glycerides or cocoa butter, is first melted and the activecomponent is dispersed homogeneously therein, as by stirring. The moltenhomogeneous mixture is then poured into convenient sized molds, allowedto cool, and thereby to solidify.

Liquid form preparations include solutions, suspensions, and emulsions,for example, water or water/propylene glycol solutions. For parenteralinjection, liquid preparations can be formulated in solution in aqueouspolyethylene glycol solution.

Aqueous solutions suitable for oral use can be prepared by dissolvingthe active component in water and adding suitable colorants, flavors,stabilizers, and thickening agents as desired. Aqueous suspensionssuitable for oral use can be made by dispersing the finely dividedactive component in water with viscous material, such as natural orsynthetic gums, resins, methylcellulose, sodium carboxymethylcellulose,and other well-known suspending agents.

Also included are solid form preparations, which are intended to beconverted, shortly before use, to liquid form preparations for oraladministration. Such liquid forms include solutions, suspensions, andemulsions. These preparations may contain, in addition to the activecomponent, colorants, flavors, stabilizers, buffers, artificial andnatural sweeteners, dispersants, thickeners, solubilizing agents, andthe like.

The pharmaceutical preparation is preferably in unit dosage form. Insuch form the preparation is subdivided into unit doses containingappropriate quantities of the active component. The unit dosage form canbe a packaged preparation, the package containing discrete quantities ofpreparation, such as packeted tablets, capsules, and powders in vials orampoules. Also, the unit dosage form can be a capsule, tablet, cachet,or lozenge itself, or it can be the appropriate number of any of thesein packaged form.

The quantity of active component in a unit dose preparation may bevaried or adjusted from 0.1 mg to 10 g, more typically 1.0 mg to 1 g,most typically 10 mg to 500 mg, according to the particular applicationand the potency of the active component. The composition can, ifdesired, also contain other compatible therapeutic or diagnostic agents.

Effective Dosages

MRI contrast agents are typically administered at a dosage of 0.1-0.3mmol/kg patient in 0.5 M solutions (Caravan, P. E. et al., Chem. Rev.1999, 99, 2293). The improved properties of the present agents allowstheir administration in amounts lower than art-recognizedaminocarboxylate-based contrast agents. Adjusting the dose to achievemaximal efficacy in humans based on the methods described above andother methods as are well-known in the art is well within thecapabilities of the ordinarily skilled artisan.

The Methods

In an exemplary embodiment, the invention provides a method forperforming a contrast enhanced imaging study on a subject. The methodincludes administering a metal complex of the invention to the subjectand acquiring an image of the subject. The complexes of the inventionare of use in a range of diagnostic imaging modalities including, butnot limited to, MRI, X-ray and CT.

The invention also provides a method for tuning a physical property of ametal complex of the invention. Exemplary physical properties that aretuned by the method of the invention include, but are not limited to,water exchange rate, rotational correlation time, in vivo residencetime, relaxivity and water solubility. In one embodiment the methodincludes preparing a parent complex that may or may not have apoly(ether) as a component of the ligand and measuring the physicalproperty of the complex. If the measured property is less than ideal,the property is adjusted by preparing an analogue of the ligand thatincludes a poly(ether), or that has a poly(ether) of a size and/orstructure different from that of the first ligand. The inventors haverecognized that, using the iterative method provided herein, it ispossible to tune and refine a range of physical properties of thechelates of the invention.

Also provided by the present invention is a method for treating apatient for metal ion overload. The method includes administering to apatient in need of such treatment an amount of a compound prepared bythe method of the invention. The amount of compound administered iseffective to reduce the metal ion load in the patient. It is well withinthe abilities of one of skill in the art to ascertain an appropriatedosage and treatment regimen for a particular patient.

In a preferred embodiment, treating the patient with a compound preparedby a method of the invention results in a greater amount of metal ionbeing removed from the patient than is removed upon treating the patientwith an identical dose of the same compound prepared by a previouslyknown method.

The materials, methods and devices of the present invention are furtherillustrated by the examples that follow. These examples are offered toillustrate, but not to limit the claimed invention.

EXAMPLES Example 1 Preparation of6-methyl-3-hydroxy-2(1H)-pyridinone-4-carboxylic acid ethyl ester(Formula 8 in Scheme 2)

6-methyl-3-hydroxy-2(1H)-pyridinone 4-carboxylic acid ethyl ester-is aknown material; however the previous preparation results in an impureproduct of low yield. Therefore, an improved procedure has beendeveloped, which is higher yielding and can be scaled up to give largequantities of the product in high purity. The details are described asfollows:

Sodium diethyloxylacetate (42.1 g, 200 mmol) was dissolved in THF (500ml) and then placed into a 1-liter 3-neck round bottom flask.Chloroacetone (16 ml, 200 mmol) was added to the mixture. After waiting10 minutes, NH₃ gas was bubbled through the reaction and AlCl₃ (2.67 g,20 mmols) was slowly and carefully added. The reaction was stirred underambient conditions for 5 days. The resulting orange solid was filtered,and taken up in 1 M HCl (500 ml) so that the pH<3. The resultingsuspension was stirred for 30 min and the precipitate filtered, washedwith distilled water and recrystallized from hot EtOH (approx 1 L) toyield colorless crystals (Yield: 15.7 g, 40%). mp 227-229° C. ¹H NMR(d₆-DMSO, 300 MHz): δ=1.24 (t, 3H, CH₃), 2.07 (s, 3H, CH₃), 4.22 (q, 2H,CH₂), 6.07 (s, 1H, CH) ppm. Anal. Calcd (Found) for C₉H₁₁O₄N: C, 54.82(55.06); H, 5.62 (5.53); N, 7.11 (7.07). EI-MS (+): m/z: 198[M+H]⁺.

Example 2 Preparation of 3-benzyloxy-4-carboxy-6-methyl-2(1H)-pyridinone(Formula 11 in Scheme 2)

6-methyl-3-hydroxy-2(1H)-pyridinone-4-carboxylic acid ethyl ester (11.8g, 60 mmol) and K₂CO₃ (9.06 g, 65 mmol) were dissolved in H₂0 (650 ml)with aid of ultra-sonication. This solution was added to a solution ofbenzyl bromide (7.8 mL, 65 mmol) in CH₂Cl₂ (500 ml) in a 2-liter 3-neckround bottom flask. Cetylpyridinium chloride (9.09 g, 30 mmol) was addedas the phase transfer catalyst for this reaction. The solution wasstirred with an overhead stirrer, at 40° C. for 1 day until the reactionwas judged to be complete by TLC. The two layers were separated, and theaqueous layer was extracted twice with CH₂Cl₂ (100 ml). The organiclayers were combined and the solvents were removed. Purification, ofthis crude product (3-benzyloxy-6-methyl-2(1H)-pyridinone-4-carboxylicacid ethyl ester (9) was possible by column chromatography, although itwas found to be more convenient to use the crude product directly in thesubsequent ester hydrolysis reaction, and purify after this step. ¹H NMR(CDCl₃, 300 MHz): δ=1.29 (t, 3H, CH₃), 2.35 (s, 3H, CH₃), 4.30 (q, 2H,CH₂), 5.26 (d, 2H, CH₂), 6.22 (s, 1H, CH), 7.35 (m, 5H, Ph) ppm. Theproduct was dissolved in a solution of KOH (16.68 g, 297 mmol) in MeOH(300 ml) and the solution heated under reflux for 24 h or until thereaction was judged to be complete by TLC. The solution was filtered andacidified to pH=1 with 6M HCl. The white solid was filtered, washed with100 mL of H₂0 and recrystallized from EtOH (400 ml) to give a whitecrystalline solid (Yield: 7.06 g, 46% from4-carboxyethylester-6-methyl-3-hydroxy-2(1H)-pyridinone). ¹H NMR(d₆-DMSO, 300 MHz): δ=2.11 (s, 3H, CH₃), 5.07 (d, 2H, CH₂), 5.97 (s, 1H,CH), 7.30 (m, 5H, Ph) ppm. Anal. Calcd (Found) for C₁₄H₁₃O₄N: C, 64.88(64.73); H, 5.02 (5.15); N, 5.41 (5.37). El-MS (+): m/z: 259[M]⁺.

Example 3 Preparation of3-benzyloxy-6-methyl-4-(2-thioxo-thiazolidine-3-carbonyl)-2(1H)-pyridinone(Formula 12 in Scheme 2)

3-benzyloxy-4-carboxy-6-methyl-2(1H)-pyridinone (5.39 g, 21.1 mmol) wasdissolved in dry THF (200 ml). Then 2-mercaptothiazoline (2.71 g, 23.2mmol) and DMAP (0.25 g, 2.0 mmol) were added and the solution stirredunder an atmosphere of N₂ for 1 h. Then dicyclohexylcarbodiimide (DCC,5.12 g, 25.3 mmol) was added in small portions at intervals split over 3hrs. The reaction was left to stir for 16 h, and then left to stand at0° C. for a further 24 h. The dicyclohexylurea (DCU) was removed byfiltration, and the solvents removed. The remaining residue wasdissolved in CH₂Cl₂, filtered and purified by a gradient flash silicacolumn (2-6% CH₃OH in CH₂Cl₂). The solvents were removed from theappropriate fractions by rotary evaporation, and the residuerecrystallized from acetone, yielding yellow crystals (3.78 g, 56%). ¹HNMR (CDCl₃, 300 MHz): δ=7.32-7.47 (m, 5H, aromatic Bn), 5.98 (s, 1H,CH), 5.34 (s, 2H, Bn CH₂), 4.34 (t, J=7.3 Hz, 2H, CH₂), 2.92 (t, J=7.3Hz, 2H, CH₂), 2.34 (s, 3H, CH₃) ppm. Anal. Calcd (Found) forC₁₇H₁₆N₂O₃S₂: C, 56.65 (56.50); H, 4.47 (4.51); N, 7.77 (7.70). FAB-MS(+): m/z: 361[M+H]⁺.

Example 4 Preparation of1-benzyl-3-benzyloxy-4-carboxy-6-methyl-2(1H)-pyridinone (Formula 49 inScheme 10)

K₂CO₃ (10.5 g, 75.4 mmol) was added to a suspension of6-methyl-3-hydroxy-2(1H)-pyridinone-4-carboxylic acid ethyl ester (5.20g, 26.4 mmol) in dry DMF (10 ml) under an atmosphere of N₂. Benzylchloride (6.96 g, 55.0 mmol) was added and the reaction stirred at 65°C. for 30 h, the progress of the reaction was monitored by TLC. Thesolvents were removed by rotary evaporation and the residue taken up inEtOAc (100 ml) and water (100 ml) The organic layer was washed withwater (5×100 ml), dried (Na2SO₄), filtered and evaporated to dryness.The crude product,1-benzyl-3-benzyloxy-6-methyl-2(1H)-pyridinone-4-carboxylic acid ethylester (Formula 10), was partially purified by column chromatography(eluent: CH₂Cl₂), and it was obtained as a light brown oil. (Crudeyield: 7.46 g, 75%). ¹H NMR (CDCl₃, 300 MHz): δ=7.10-7.51 (m, 10H, Bn),6.18 (s, 1H, HOPO), 5.36 (s, 2H, Bn-CH₂), 5.31 (s, 2H, Bn-CH₂), 4.30 (q,J=7.1 Hz, 2H, CH₂), 2.26 (s, 3H, HOPO CH₃), 1.28 (t, J=6.9 Hz, 3H, CH₃Et ester) ppm. FAB-MS (+): m/z: 378.2[M+H]⁺. The resulting oil wasdissolved in a solution of KOH (2.3 g, 40 mmol) in MeOH (200 ml).

The reaction was left stirring for 2 days, by which time TLC indicatedthat the reaction had gone to completion. The solvent was removed, andthe residue dissolved in distilled water. The pH of the solution waslowered to 1 by addition of 6.0M HCl, to yield a white precipitate whichwas filtered and dried (Yield: 6.27 g, 68% based on6-methyl-3-hydroxy-2(1H)-pyridinone-4-carboxylic acid ethyl ester). ¹HNMR (CDCl₃, 400 MHz): δ=7.30-7.44 (m, 10H, Bn), 6.58 (s, 1H, HOPO), 5.63(s, 2H, Bn-CH₂), 5.37 (s, 2H, Bn-CH₂), 2.30 (s, 3H, HOPO CH₃) ppm. Anal.Calcd (Found) for C₂₁H₁₉NO₄.0.4H₂O: C, 70.73 (72.19); H, 5.60 (5.66); N,3.93 (3.94). FAB-MS (+): m/z: 350[M+1]⁺. Crystals suitable for X-raydiffraction were obtained by slow evaporation of a solution of AcOEt.

Example 5 Preparation of1-benzyl-3-benzyloxy-6-methyl-4-(2-thioxo-thiazolidine-3-carbonyl)-2(1H)-pyridinone(Formula 50 in Scheme 10)

1-benzyl-3-benzyloxy-4-carboxy-6-methyl-2(1H)-pyridinone (5.36 g, 15.3mmol) was dissolved in CH₂Cl₂ (100 ml) under an atmosphere of nitrogen.Then 2-mercaptothiazoline (1.88 g, 16.1 mmol) and dimethylaminopyridine(DMAP, 0.16 g, 1.0 mmol) were added and the solution stirred for 1 h.Dicyclohexylcarbodiimide (DCC, 3.45 g, 16.9 mmol) was then added insmall portions and the reaction mixture stirred for 16 h, and thenallowed to stand for 24 h at 0° C. The reaction mixture was filtered andthe solvent removed by evaporation. EtOAc (10 ml) was added, and thesolution filtered. This last step was repeated 3 times in order toremove the dicyclohexylurea (DCU), yielding a yellow oil. The productwas purified by flash column chromatography (silica, CH₂Cl₂ as eluent).The solvents were removed by rotary evaporation, yielding a viscousyellow oil, which was recrystallized from acetone to give a yellowcrystalline solid (Yield: 5.10 g, 74%). ¹H NMR (CDCl₃, 400 MHz):δ=7.12-7.49 (m, 10H, Bn), 5.98 (s, 1H, HOPO), 5.38 (s, 2H, Bn-CH₂), 5.34(s, 2H, Bn-CH₂), 4.59 (t, J=7.3 Hz, 2H, thiaz), 2.90 (t, 2H, J=7.3 Hz,thiaz), 2.27 (s, 3H, HOPO CH₃) ppm. Anal. Calcd (Found) forC₂₄H₂₂N₂O₃S₂: C, 63.98 (64.04); H, 4.92 (5.02); N, 6.22 (6.03). FAB-MS(+): m/z: 451 [M+H]⁺.

Example 6 Preparation of4-(2-ethanolcarbamoyl)-3-hydroxy-6-methyl-2(1H)-pyridinone (Formula 10C)(1) Preparation of 3-benzyloxy-4-(2-ethanolcarbamoyl)-6-methyl-2( 1H)-pyridinone (10B)

Ethanolamine (0.044 ml, 0.672 mmol) was added to a solution of3-benzyloxy-6-methyl-4-(2-thioxo-thiazolidine-3-carbonyl)-2(1H)-pyridinone (0.200 g, 0.577 mmol) in CH₂Cl₂ (15 ml). The resultingsolution was stirred in the dark for 10 h under ambient conditionsduring which time the solution turned from yellow to colorless. Thissolution was purified by a gradient flash silica column (2-8% CH3OH inCH2Cl2), and the solvent removed to yield a white solid, (Yield: 0.161g, 97%). ¹H NMR (CDCl₃, 300 MHz): δ=8.21 (t, 1H, NH), 7.38-7.46 (m, 5H,Bn), 6.64 (s, 1H, HOPO-H), 5.38 (s, 6H, CH₂), 3.61 (t, J=4.8 Hz, 2H,CH₂), 3.38 (q, J=4.8 Hz, 2H, CH₂), 2.33 (s, 3H, CH₃) ppm. Anal. Calcd(Found) for C₁₆H₁₈N₂O₄: C, 63.56 (63.21); H, 6.00 (6.12); N, 9.27 (8.99)ES-MS (+): m/z: 302[M]⁺.

(2) Preparation of4-(2-ethanolcarbamoyl)-3-hydroxy-6-methyl-2(1H)-pyridinone (10C)

5% Pd on C (0.100 g) was added to a solution of3-benzyloxy-4(2-ethanolcarbamoyl)-6-methyl-2(1H)-pyridinone (0.100 g,0.348 mmol) in AcOH (5 ml) and MeOH (5 ml) and the mixture stirred underan atmosphere of H₂ under ambient conditions for 12 h. The reaction wasthen filtered and the solvent removed. The residue was thenre-precipitated from AcOH/MeCN to yield a light tan colored solid.(Yield: 0.042 g, 61%). ¹H NMR (D₂O, 500 MHz): δ=6.44 (s, 1H, HOPO-H),3.79 (t, J=5.3 Hz, 2H, CH₂), 3.56 (t, J=5.3 Hz, 2H, CH₂), 2.27 (s, 3H,CH₃) ppm. Anal. Calcd (Found) for C₉H₁₂N₂O₄: C, 50.94 (50.60); H, 5.70(5.72); N, 13.20 (13.29) ppm. El-MS (+): m/z: 212[M]⁺.

Example 7 Preparation ofN,N,N,-tris[3-hydroxy-6-methyl-2(1H)-pyridinone-4-carboxylamideethyl]amine(TREN-6-Me-3,2-HOPO, Formula 14 in Scheme 2) (1) Preparation ofN,N,N,-tris[3-benzyloxy-6-methyl-2(1H)-pyridinone-4-carboxylamideethyl]amine(Formula 13 in Scheme 2)

3-benzyloxy-6-methyl-4-(2-thioxo-thiazolidine-3-carbonyl)-2(1H)-pyridinone(1.5 g, 4.3 mmol) was dissolved in CH₂Cl₂ (30 ml). To this were addedsuccessive portions of tris(2-aminoethylamine) with a 4 h delay betweensubsequent additions until the solution turned from yellow to colorless.The solution was purified by two separate elutions down a silica column(solvent: CH₂Cl₂ with increasing gradient of MeOH from 2% to 8%).Removal of the solvent afforded a white glassy solid. (Yield: 0.91 g,76%). ¹H NMR (d₆-DMSO, 400 MHz): δ=8.12 (t, J=7.2 Hz, 3H, NH), 7.20-7.37(m, 15H, Bn), 5.95 (s, 3H, HOPO-H), 5.12 (s, 6H, Bn CH₂), 3.14 (m, 6H,TREN CH₂), 2.42 (m, 6H, TREN CH₂), 2.08 (s, 9H, CH₃) ppm. Anal. Calcd(Found) for C₄₈H₅₁N₇O₉: C, 66.27 (65.97); H, 5.91 (5.91); N, 11.27(11.17). FAB-MS (+): m/z: 870[M+H]⁺.

(2) Preparation ofN,N,N,-tris[3-hydroxy-6-methyl-2(1H)-pyridinone-4-carboxylamideethyl]amine(TREN-6-Me-3,2-HOPO, Formula 14 in Scheme 2)

5% Pd on carbon (0.235 g) was added to a solution ofN,N,N,-tris[3-benzyloxy-6-methyl-2(1H)-pyridinone-4-carboxylamideethyl]amine(0.360 g, 0.414 mmol) in AcOH (10 ml) and H₂O (5 ml) and the mixturestirred under an atmosphere of H₂ under ambient conditions for 2 h. Thereaction was then filtered and the solvent removed. The remainingresidue converted to the Cl⁻ salt by dissolving in MeOH (10 ml) and onedrop of conc. HCl and the solvent removed (×3). Excess HCl was thenremoved by dissolving the residue in MeOH (10 ml) and removing thesolvent under reduced pressure (×3). The remaining residue was taken upin MeOH (2 ml) and added to a rapidly stirring solution of Et₂O (200 ml)to afford a white precipitate, which was filtered and dried undervacuum. (Yield: 0.225 g, 78%). ¹H NMR (D₂O, 500 MHz): δ=5.87 (s, 3H,HOPO-H), 3.79 (m, 6H, TREN CH₂), 3.61 (m, 6H, TREN CH₂), 1.96 (s, 9H,CH₃) ppm. Anal. Calcd (Found) for TREN-6-Me-3,2-HOPO-HCl.3H₂O,C₂₇H₃₄N₇O₈Cl.3H₂O: C, 46.39 (46.09); H, 5.91 (5.82); N, 14.02 (13.80)ppm. FAB-MS (+): m/z: 600[M]⁺.

Example 8 Preparation of the Gadolinium(III) Ion Complex withN,N,N,-tris[3-hydroxy-6-methyl-2(1H)-pyridinone-4-carboxylamideethyl]amine(Gd-TREN-6-Me-3,2-HOPO) (Formula 15, Scheme 2)

GdCl₃.6H₂O (0.0532 g, 0.136 mmol) was added to a solution ofTREN-6-Me-HOPO (0.100 g, 0.143 mmol) in H₂O (10 ml) with an excess ofpyridine and the solution heated to reflux for 2 h. A microcrystallinewhite solid precipitated from solution, which was filtered and driedunder vacuum. (Yield: 0.080 g, 70%). Anal; Calcd (Found) forGd(TREN-6-Me-3,2-HOPO)(H₂O)₂.H₂O, C₂₇H₃₈N₇O₁₀Gd.H₂O: C, 39.05 (39.37);H, 4.87 (4.42); N, 13.80 (13.37); Gd, 15.49 (15.35) ppm.

Example 9 Preparation of2,3-bis-benzyloxy-1,4-bis(2-thioxo-thiazolidine-3-carbonyl)-benzene(BnTAMdiThiaz) (Formula 23 in Scheme 3)

Oxalyl Chloride (14 ml, 160 mmol) and DMF (1 drop) was added to asuspension of 2,3-Bis-benzyloxy-terephthalic acid (20 g, 52 mmol) in drytoluene (250 ml). After 24 h of stirring under an atmosphere of N₂, alight brown solution was obtained. The solvent was removed by rotaryevaporation, and the residue dissolved in dry THF (200 ml). The solutionwas cooled to −35° C., and a solution of 2-mercaptothiazoline (12.6 g,104 mmol) and NEt₃ (10.1 g, 100 mmol) in THF (100 ml) was added dropwiseover a period of 2 h. The solution was filtered at 0° C. to removeNEt₃.HCl. The solvents were removed, and the residue dissolved in CH₂Cl₂(50 ml) and filtered through a silica plug to yield a bight yellowsolution. The solvent was removed and the product was recrystallizedfrom acetone (500 ml) to afford a bright yellow prisms. (Yield: 22.9 g,76%). ¹H NMR (CDCl₃, 400 MHz): δ=7.20-7.36 (m, 12H, aromatic H), 5.08(s, 4H, Bn CH₂), 4.32 (t, J=7.3 Hz, 4H, CH₂), 2.96 (t, J=7.3 Hz, 4H,CH₂) ppm. Anal. Calcd (Found) for C₂₈H₂₄N₂O₄S₄: C, 57.91 (57.92); H,4.17 (4.13); N, 4.82 (4.74). FAB-MS (+): m/z: 581 [M+H]⁺.

Example 10 Preparation of TREN-1-Bn-6-Me-3,2-HOPO-TAM (19C) (1)Preparation of benzyl protected TREN-1-Bn-6-Me-3,2-HOPO-TAM-thiazolide(1 9A)

A solution of1-benzyl-3-benzyloxy-6-methyl-4-(2-thioxo-thiazolidine-3-carbonyl)-2(1H)-pyridinone(1.5 g, 3.3 mmol) in CH₂Cl₂ (100 ml) was added dropwise to a rapidlystirring solution of tris(2-aminoethylamine) (TREN, 0.219 g, 1.5 mmol)in CH₂Cl₂ (100 ml). The resulting solution was purified by a gradientflash silica column (2-10% CH₃OH and 1% Et₃N in CH₂Cl₂) This compound,TREN-(1-Bn-6-Me-3,2-HOPO)₂ (Formula 7, R₁=benzyl, R₂=methyl, c=1, p=2)was dissolved in CH₂Cl₂ and added dropwise to a rapidly stirringsolution of BnTAMdiThiaz (Formula 8, 14 g, 24 mmol) in CH₂Cl₂ (100 ml)over 2 h. The product was purified by a gradient flash silica column(2-5% CH₃OH in CH₂Cl₂), and obtained as a yellow foam. (Yield: 1.39 g,73%). ¹H NMR (CDCl₃, 400 MHz): δ=8.30 (br s, 1H, TAM NH), 7.94 (br s,2H, HOPO NH), 7.85 (d, 1H, TAM CH), 7.27 (br s, 1H, TAM NH), 7.09-7.38(m, 31H, aromatic H), 6.58 (s, 2H, HOPO-H), 5.35 (s, 8H, HOPO Bn CH₂),5.12 (s, 2H, TAM Bn CH₂), 5.09 (s, 2H, TAM Bn CH₂), 4.37 (t, J=7.3 Hz,2H, thiazolide CH₂), 3.38 (m, 6H, TREN CH₂), 2.93 (t, J=7.3 Hz, 2H,thiazolide CH₂), 2.37 (m, 6H, TREN CH₂), 2.25 (s, 6H, HOPO CH₃) ppm.Anal. Calcd (Found) for C₇₃H₇₁N₇O₁₀S₂.3.5H₂O: C, 65.77 (65.52); H, 5.86(5.56); N, 7.36 (7.04). FAB-MS (+): m/z: 1270.5 [M+1]⁺.

(2) Benzyl Protected TREN-1-Bn-6-Me-3,2-HOPO-TAM (19B)

Excess methylamine (solution in H₂O) was shaken with a solution ofbenzyl protected TREN-1-Bn-6-Me-3,2-HOPO-TAM-thiazolide (1.12 g, 0.883mmol) in CH₂Cl₂ (100 ml), and the solution tuned from yellow tocolorless within a few seconds. The product was purified by a gradientflash silica column (2-10% CH₃OH in CH₂Cl₂) to yield a white foam afterremoval of solvents. (Yield: 0.76 g, 73%). ¹H NMR (CDCl₃, 400 MHz):δ=8.20-8.25 (m, 4H, NH), 7.05-7.37 (m, 32H, aromatic H), 6.18 (s, 2H,HOPO-H), 5.28 (s, 4H, HOPO Bn CH₂), 5.18 (s, 4H, HOPO Bn CH₂), 5.03 (s,2H, TAM Bn CH₂), 5.00 (s, 2H, TAM Bn CH₂), 3.29 (m, 6H, TREN CH₂), 2.71(d, 3H, TAM CH₃), 2.51 (m, 6H, TREN CH₂), 2.17 (s, 6H, HOPO CH₃) ppm.Anal. Calcd (Found) for C₇₁H₇₁N₇₀O₁₀.2H₂O: C, 69.99 (69.96); H, 6.20(6.03); N, 8.05 (7.88). FAB-MS (+): m/z: 1182.6 [M+1]⁺.

(3) TREN-1-Bn-6-Me-3,2-HOPO-TAM (19C)

10% Pd on carbon (0.235 g) was added to a solution of benzyl protectedTREN-1-Bn-6-Me-HOPO-TAM (0.540 g, 0.457 mmol) in AcOH (10 ml) and H₂O (5ml) and the mixture stirred under an atmosphere of H₂ under ambientconditions for 48 h. The reaction was then filtered and the solventremoved. The remaining residue was converted to the Cl⁻ salt bydissolving in MeOH (10 ml) and one drop of conc. HCl and the solventremoved (×3). Excess HCl was removed by dissolving the residue in MeOH(10 ml) and removing the solvent under reduced pressure (×3). Theremaining residue was taken up in MeOH (2 ml) and added to a rapidlystirring solution of Et₂O (200 ml) to afford a white precipitate whichwas filtered and dried under vacuum. (Yield: 0.324 g, 79%). ¹H NMR(d₆-DMSO, 300 MHz): δ=9.11 (br s, 1H, NH), 8.90 (br s, 1H, NH), 8.68 (brs, 2H, NH), 7.04-7.32 (m, 12H, aromatic H), 6.36 (s, 2H, HOPO-H), 5.27(s, 4H, Bn CH₂), 3.72 (m, 6H, TREN CH₂), 3.46 (m, 6H, TREN CH₂), 2.79(d, J=3.3 Hz, 3H, CH₃) ppm. Anal. Calcd (Found) forC₄₃H₄₇N₇O₁₀.HCl.3H₂O: C, 56.61 (56.60); H, 5.97 (5.89); N, 10.75(10.46). FAB-MS (+): m/z: 822 [M+H]⁺.

Example 11 Preparation of the Gadolinium(III) Ion Complex withTREN-1-Bn-6-Me-3,2-HOPO-TAM (Formula 19 in Scheme 2)

Excess pyridine was added to a solution of GdCl₃.6H₂O(0.040 g, 0.109mmol) and TREN-1-Bn-6-Me-HOPO-TAM (0.100 g, 0.114 mmol) in H₂O (10 ml).The resulting pale yellow solution was heated under reflux for 2 h,during which time a cream precipitate developed. The solvent was removedand the residue suspended in iPrOH, sonicated and filtered three times.The resulting light brown solid was dried under vacuum. (Yield: 0.114 g,95%). Anal. Calcd (Found) for GdH(TREN-1-Bn-6-Me-HOPO-TAM)(H₂O)₂.2H₂OC₄₃H₅₁N₇O₁₂Gd.2H₂O: C, 49.32 (48.95); H, 4.90 (4.09); N, 9.36 (8.91).FAB-MS (−): m/z: 975.2 [M]⁻.

Example 12 Preparation of TREN-1-Me-3,2-HOPO-TAM-DME (27B) (1)Preparation of Benzyl protected TREN-1-Me-HOPO-3,2-TAM-DME (Formula 27in Scheme 3, R═—CH₂CH₂N(CH₃)₂)

1,1-dimethylethyldiamine (0.11 ml, 1.00 mmol) was added to a solution ofTREN-Me-3,2-HOPO-TAM-Thiaz (1.00 g, 0.973 mmol) in CH₂Cl₂ (30 ml). Theresulting mixture was left to stir under ambient conditions for 8 h,during which time the color changed from yellow to clear. The solventwas removed by rotary evaporation and the residue was purified by agradient flash silica column (2-10% CH₃OH and 1% Et₃N in CH₂Cl₂). Thesolvents were removed to afford a white solid, which was taken up inMeOH (10 ml) and evaporated once again (repeated twice) in order toremove excess NEt₃ (Yield: 0.84 g, 85%). ¹H NMR (CDCl₃ with a trace ofNEt₃ added to improve signal of DME protons, 500 MHz): δ=8.04 (t, 1H,TAM amide NH), 7.80 (m, 3H, TAM-H and HOPO amide NH), 7.68 (d, J=8.4 Hz,1H, TAM-H), 7.57 (br t, 1H, TAM amide NH) 7.28-7.41 (m, 20H, Bn), 7.06(d, J=7.2 Hz, 2H, HOPO-H), 6.61 (d, J=7.2 Hz, 2H, HOPO-H), 5.29 (s, 4H,HOPO Bn CH₂), 5.15 (s, 2H, TAM Bn CH₂), 5.06 (s, 2H, TAM Bn CH₂), 3.57(s, 6H, HOPO CH₃), 3.45 (q, 2H, DME CH₂), 3.10 (m, 6H, TREN CH₂), 2.30(m, 8H, TREN CH₂ and DME CH₂) 2.13 (s, 6H, DME CH₃) ppm. Anal. Calcd(Found) for C₆₀H₆₆N₈O₁₀.2.5H₂O: C, 65.29 (65.28); H, 6.44 (6.31); N,10.15 (10.01). ES-MS (+): m/z: 1059.3[M+H]⁺.

(2) Preparation of TREN-1-Me-3,2-HOPO-TAM-DME (27B)

10% Pd on carbon (0.10 g) was added to a solution of benzyl protectedTREN-1-Me-HOPO-TAM-DME (0.826 g, 0.781 mmol) in AcOH (10 ml). Theresulting mixture was stirred under an atmosphere of H₂ under ambientconditions for 2 days. The Pd/C was removed by filtration, and thesolvent was removed by rotary evaporation. The remaining residue wasconverted to the Cl⁻ salt by dissolving in MeOH (10 ml) and one drop ofconc. HCl and the solvent removed (×3). Excess HCl was then removed bydissolving the residue in MeOH (10 ml) and removing the solvent underreduced pressure (×3). The remaining residue was taken up in MeOH (2 ml)and added to a rapidly stirring solution of Et₂O (200 ml) to afford awhite precipitate which was filtered and dried under vacuum. (Yield:0.640 g, 95%). ¹H NMR (D₂O, 500 MHz): δ=7.01 (d, J=10.9 Hz, 1H, TAM-H),6.97 (d, J=10.9 Hz, 1H, TAM-H), 6.88 (d, J=9.1 Hz, 2H, HOPO-H), 6.23 (d,J=9.10Hz, 2H, HOPO-H), 3.87 (m, 6H, TREN CH₂), 3.82 (t, J=7.4 Hz, 2H,DME CH₂), 3.70 (m, 6H, TREN CH₂), 3.48 (s, 6H, DME CH₃), 3.43 (t, J=7.4Hz, 2H, DME CH₂), 2.97 (s, 6H, TREN CH₃), ppm. Anal. Calcd (Found) forC₃₂H₄₂N₈O₁₀.2HCl3.5H₂O: C, 46.05 (46.15); H, 6.16 (6.29); N, 13.42(13.33). FAB-MS (+): m/z: 699[M+H]⁺.

Example 13 Preparation of the Gadolinium(III) Ion Complex withTREN-1-Me-HOPO-TAM-DME (27C)

GdCl₃.6H₂O (0.073 g, 0.195 mmol) was added to a solution ofTREN-HOPO-TAM-DME (0.150 g, 0.195 mmol) in H₂0 (10 ml) and the solutionstirred for 1 h. Imidazole (0.093 g, 1.17 mmol) was added and thesolution heated to reflux for 2 h during which time a white solidprecipitated from solution. The solvents were evaporated and the residuesuspended in iPrOH, sonicated, filtered, and dried under vacuum. (Yield:0.128 g, 74%). Anal. Calcd (Found) for Gd(L)(H₂O)₂, C₃₂H₄₃N₈O₁₂Gd: C,43.28 (42.97); H, 4.88 (4.87); N, 12.60 (12.33); Gd, 17.69 (18.14).ES-MS (+): m/z: 853 [M+H]⁺. The UV/Vis spectrum was identical to that ofGd-TREN-1-Me-HOPO-TAM at neutral pH.

Example 14 Preparation of TREN-1-Me-3,2-HOPO-TAM-PEG-5000 (Formula 47A)(1) Preparation of Benzyl protected TREN-1-Me-3,2-HOPO-TAM-Thiazolide(Formula 25 in Scheme 3)

A solution of TREN(1-Me-HOPO)₂ (3.00 g, 4.77 mmol) in CH₂Cl₂ (150 ml)was added dropwise to a solution of BnTAMdiThiaz (20.77 g, 47.7 mmol) inCH₂Cl₂ (250 ml) over a period of 1 h. The resulting solution was thenstirred for a further 8 h before purified (twice) by a gradient flashsilica column (0-4% CH₃OH in CH₂Cl₂)). The solvents were removed underreduced pressure to afford a yellow foam. (Yield: 3.56 g, 68%). ¹H NMR(CDCl₃, 500 MHz): δ=7.82 (t, J=5.3 Hz, 2H, HOPO amide NH), 7.78 (d,J=8.2 Hz, 1H, TAM-H), 7.22-7.41 (m, 20H, Bn), 7.18 (d, J=8.2 Hz, 1H,TAM-H), 7.08 (d, J=7.2 Hz, 2H, HOPO-H), 6.65 (d, J=7.2 Hz, 2H, HOPO-H),5.30 (s, 4H, HOPO Bn CH₂), 5.11 (s, 2H, TAM Bn CH₂), 5.06 (s, 2H, TAM BnCH₂), 4.38 (t, J=7.3 Hz, 2H, TAM thiaz), 3.57 (s, 6H, CH₃), 3.12 (m, 6H,TREN CH₂), 2.93 (t, J=7.3 Hz, 2H, TAM thiaz), 2.29 (m, 6H, TREN CH₂)ppm. Anal. Calcd (Found) for C₅₉H₅₉N₇O₁₀S₂: C, 65.00 (64.68); H, 5.45(5.30); N, 8.99 (8.73). FAB-MS (+): m/z: 1090.5[M+H]⁺.

(2) Preparation of Benzyl Protected TREN-1-Me-HOPO-TAM-PEG-5000 (Formula27 in Scheme 3, R═(CH₂CH₂O)_(n)CH₃, average n=121)

PEG-5000 monoamine (1.85 g, 0.370 mmol) was added to a solution ofbenzyl protected TREN-1-Me-HOPO-TAM-thiazolide (0.4 g, 0.377 mmol) indry CH₂Cl₂ (5 ml). The resulting solution was left stirring for severaldays at 313K under a nitrogen atmosphere. After 4 days, a few drops ofNet3 and a catalytic amount of dimethylaminopyridine (DMAP) were added.After TLC indicated that the reaction was complete, the solvent wasremoved and the residue was purified by a gradient flash silica column(2-10% CH₃OH in CH₂Cl₂)) and sephadex LH-20 (solvent: MeOH) columnchromatography until pure by GPC analysis. (Yield: 1.95 g, 91%). ¹H NMR(CDCl₃ with AcOH added, 500 MHz): δ=8.12 (m, 4H, NH's), 7.70 (d, J=8.3Hz, 1H, TAM-H), 7.54 (d, J=8.3 Hz, 1H, TAM-H), 7.26-7.37 (m, 20H, Bn),7.06 (d, J=7.5 Hz, 2H, HOPO-H), 6.53 (d, J=7.5 Hz, 2H, HOPO-H), 5.30 (s,4H, HOPO Bn CH₂), 5.12 (s, 2H, TAM Bn CH₂), 5.09 ppm. All other peaksare obscured by PEG and AcOH peaks in the between 2 and 4 ppm. MS-ES(+):m/z: 1582 (average mass), [M]⁴⁺ with 121 —(CH₂CH₂O)— units. Peaks inthis region were separated by 11 mass units, as expected for a PEGcompound.

(3) Preparation of TREN-1-Me-HOPO-TAM-PEG-5000 (47A)

5% Pd on C (0.20 g) was added to a solution of benzyl protectedTREN-1-Me-HOPO-TAM-PEG-5000 (2.00 g, 0.334 mmol) in AcOH (20 ml). Theresulting mixture was stirred under an atmosphere of H₂ under ambientconditions for 3 days. The Pd/C was removed by filtration, and thesolvent was removed by rotary evaporation. The remaining residue wasconverted to the Cl⁻ salt by dissolving in MeOH (10 ml) and one drop ofconc. HCl and the solvent removed (×3). Excess HCl was then removed bydissolving the residue in MeOH (10 ml) and removing the solvent underreduced pressure (×3). The remaining residue was taken up in MeOH (2 ml)and added to a rapidly stirring solution of Et₂O (200 ml) to afford awhite precipitate, which was filtered and dried under vacuum. (Yield:1.95 g, 91%). ¹H NMR (D₂O, 500 MHz): δ=6.96 (d, 1H, TAM-H), 6.91 (d, 1H,TAM-H), 6.73 (d, J=7.2 Hz, 2H, HOPO-H), 6.04 (d, J=7.2 Hz, 2H, HOPO-H),3.2-2.8 (m, large integral, PEG and other protons) ppm. Anal. Calcd(Found) with 121 —(CH₂CH₂O)— units for C₂₇₁H₅₂ON₇O₁₃₁Cl: C, 54.17(53.76); H, 8.72 (8.77); N, 1.63 (1.35).

Example 15 Preparation of the Gadolinium(III) Ion Complex withTREN-1-Me-3,2-HOPO-TAM-PEG-5000 (formula 47)

GdCl₃.6H₂O (0.0146 g, 0.0392 mmol) was added to a solution ofTREN-1-Me-HOPO-TAM-PEG-5000 (0.220 g, 0.0392 mmol) in H₂O (10 ml). Anexcess of aqueous ammonia was added and the solution was heated toreflux for 2 h. The solution was filtered and the solvents removed fromthe filtrate by rotary evaporation. The product was purified by elutiondown a Sephadex LH-20 column, after which the solution was added to arapidly stirring solution of Et₂O (200 ml) to afford a pale yellowprecipitate, which was filtered off and dried under vacuum. (Yield:0.196 g, 87%). Anal. Calcd (Found) with 121 —(CH₂CH₂O)— units for[Gd(L)].NH₄.3NH₄Cl, C₂₇₁H₅₃₁N₁₁O₁₃₁GdCl₃: C, 51.63 (51.53); H, 8.49(8.91); N, 2.44 (2.65); Gd, 2.49 (2.89). MS-ES-TOF (−): m/z: 3093(median mass), both the mass and the isotope distribution correspond to[Gd(LH)+2Cl]²⁻ with 121 —(CH₂CH₂O)— units. The UV/vis spectrum of thiscompound was identical to that of Gd-TREN-1-Me-HOPO-TAM at neutral pH.

Example 16 Preparation of TREN-1-Me-3,2-HOPO-TAM-PEG-2000 (46A) (1)Preparation of TAM-Thiazolide-PEG-2000 (Formula 26 in Scheme 3,R=(CH₂CH₂O)_(n)CH₃, average n=42)

A solution of PEG-2000-monoamine (1.0 g, 0.5 mmol) in CH₂Cl₂ (50 ml) wasadded drop wise to a rapidly stirring solution of BnTAMdiThiaz (10.0 g,17.5 mmol) in CH₂Cl₂ (150 ml) under an atmosphere of N₂. After 2 days,the solution was reduced in volume and purified by a gradient flashsilica column (2-10% CH₃OH in CH₂Cl₂). The first yellow band wasunreacted BnTAMdiThiaz, and was saved for future use. The second yellowband was the desired product: the solvents were removed to yield a thickyellow oil, which solidified on standing over 4 h (Yield: 1.08 g, 87%).¹H NMR (d₆-DMSO, 400 MHz): δ=8.06 (br s, 1H, NH), 7.88 (d, J=8.3 Hz, 1H,TAM CH), 7.33-7.38 (m, 10H, aromatic H), 7.20 (d, J=8.3 Hz, 1H, TAM CH),5.10 (s, 4H, Bn CH₂), 4.37 (t, J=6.8 Hz, 2H, thiazolide CH₂), 3.55-3.70(m, large integral, PEG H), 2.94 (t, J=6.8 Hz, 2H, thiazolide CH₂).Anal. Calcd (Found) with 42 —(CH₂CH₂O)— units for C₁₁₀H₁₉₂N₂O₄₆S₂: C,56.39 (56.29); H, 8.26 (8.19); N, 1.20 (1.26). MS-ES (+): m/z: 1172(average mass), [M]²⁺ with 42 —(CH₂CH₂O)— units. Peaks in this regionare separated by 22 mass units, as expected for a PEG compound.

(2) Preparation of Benzyl protected TREN-1-Me-3,2-HOPO-TAM-PEG-2000(Formula 27 in Scheme 3, R═(CH₂CH₂O)_(n)CH₃, average n=42)

TAM-thiazolide-PEG-2000 (1.0 g, 0.42 mmol) was added to a solution ofTREN(1-Me-HOPO)₂ (0.341 g, 0.54 mmol) in CH₂Cl₂ (20 ml). The resultingyellow solution was stirred under an atmosphere of N₂ until, after 2days, the solution turned colorless. The product was purified by agradient flash silica column (2-8% CH₃OH in CH₂Cl₂). The solvents wereremoved to yield a white solid (Yield: 0.91 g, 85%). ¹H NMR (d6-DMSO,500 MHz): δ=8.36 (t, 1H, TAM amide NH), 8.18 (t, 1H, TAM amide NH), 8.15(t, 2H, HOPO amide NH), 7.47 (d, J=8.9 Hz, 2H, HOPO CH), 7.26-7.40 (m,22H, aromatic H), 6.23 (d, J=8.9 Hz, 2H, HOPO CH), 5.18 (s, 4H, HOPO BnCH₂), 5.05 (s, 2H, TAM Bn CH₂), 5.03 (s, 2H, TAM Bn CH₂), 3.17-3.66 (m,PEG and TREN H) ppm. Anal. Calcd (Found) with 42 —(CH₂CH₂O)— units forC₁₄₁H₂₂₈N₇O₅₂: C, 59.35 (59.09); H, 8.05 (8.30); N, 3.44 (3.34). MS-ES(+): m/z: 1427 (average mass), [M²⁺+1] with 42 —(CH₂CH₂O)— units. Peaksin this region are separated by 22 mass units, as expected for a PEGcompound.

(3) Preparation of TREN-1-Me-3,2-HOPO-TAM-PEG-2000 (46A)

5% Pd on carbon (0.15 g) was added to a solution of benzyl protectedTREN-1-Me-3,2-HOPO-TAM-PEG-2000 (0.86 g, 0.167 mmol) in AcOH (10 ml).The resulting mixture was stirred under an atmosphere of H₂ underambient conditions for 3 days. The Pd/C was removed by filtration, andthe solvent was removed by rotary evaporation. The remaining residue wasconverted to the Cl⁻ salt by dissolving in MeOH (10 ml) and one drop ofconc. HCl and the solvent removed (×3). Excess HCl was then removed bydissolving the residue in MeOH (10 ml) and removing the solvent underreduced pressure (×3). The remaining residue was taken up in MeOH (2 ml)and added to a rapidly stirring solution of Et₂O (200 ml) to afford awhite precipitate which was filtered and dried under vacuum. (Yield:0.65 g, 85%). ¹H NMR (d₆-DMSO, 500 MHz): δ=9.09 (br s, 1H, TAM amideNH), 8.96 (br s, 1H, TAM amide NH), 8.66 (br s, 1H, HOPO amide NH), 7.33(d, 1H, TAM-H), 7.31 (d, 1H, TAM-H), 7.11 (d, J=7.2 Hz, 2H, HOPO-H),6.42 (d, J=7.2 Hz, 2H, HOPO-H), 3.22-3.71 (m, large integral, PEG andTREN H) ppm. Anal. Calcd (Found) with 42 —(CH₂CH₂O)— units forC₁₁₃H₂₀₃N₇O₅₂HCl.3H₂O: C, 52.56 (52.27); H, 8.20 (8.00); N, 3.80 (3.96).MS-ES (+): m/z: 1246 (average mass), [M]²⁺ with 42 —(CH₂CH₂O)— units.Peaks in this region are separated by 22 mass units, as expected for aPEG compound.

Example 17 Preparation of the Gadolinium(III) Ion Complex withTREN-1-Me-3,2-HOPO-TAM-PEG-2000 (Formula 46)

GdCl₃.6H₂O (0.043 g, 0.135 mmol) was added to a solution ofTREN-1-Me-3,2-HOPO-TAM-PEG-2000 (0.35 g, 0.135 mmol) in H₂0 (10 ml). Anexcess of aqueous ammonia was added and the solution was heated toreflux for 2 h. The solution was filtered and the solvents removed fromthe filtrate by rotary evaporation. The product was purified by elutiondown a Sephadex LH-20 column, after which the solution was added to arapidly stirring solution of Et₂O (200 ml) to afford a pale yellowprecipitate, which was filtered off and dried under vacuum. (Yield:0.297 g, 84%). Anal. Calcd (Found) with 42 —(CH₂CH₂O)— units for[Gd(L)].NH₄ NH₄Cl, C₁₁₃H₂₀₇N₉O₅₂GdCl: C, 49.96 (49.72); H, 7.68 (7.48);N, 4.63 (4.19); Gd, 5.79 (5.55). MS-ES-TOF (−): m/z: 2681 (median mass),both the mass and the isotope distribution correspond to [Gd(LH)+Cl]⁻with 42 —(CH₂CH₂O)— units.

Example 18 Preparation of TREN-1-Me-3,2-HOPO-TAM-O₂—NH₂ (27D) (1)Preparation of Benzyl protected TREN-1-Me-3,2-HOPO-TAM-O₂—NH₂ (Formula27 in Scheme 3, R═(CH₂CH₂O)₂CH₂CH₂NH₂)

A solution of TREN-1-Me-3,2-HOPO-TAM-thiazolide (1.50 g, 1.41 mmol) inCH₂Cl₂ (100 ml) was added to a rapidly stirring solution of solution of2-[2-(2-Amino-ethoxy)-ethoxy]-ethylamine (10.46 g, 70.6 mmol) in CH₂Cl₂(200 ml). The resulting clear solution was purified by a gradient flashsilica column (2-10% CH₃OH and 1% Et₃N in CH₂Cl₂)The solvents wereremoved to yield a colorless oil. (Yield: 0.96 g, 61%). ¹H NMR (d₆-DMSO,400 MHz): δ=8.37 (br s, 1H, TAM NH), 8.18 (br s, 1H, TAM NH), 8.14 (brs, 2H, HOPO NH), 7.46 (d, J=6.8 Hz, HOPO H), 7.25-7.39 (m, 22H, aromaticH), 6.22 (d, J=6.9 Hz, HOPO H), 5.17 (s, 4H, Bn CH₂), 5.02 (s, 4H, BnCH₂), 3.18-3.54 (m, 18H, CH₂ and HOPO CH₃) ppm. Anal. Calcd (Found) forC₆₂H₇₀N₈O₁₂.2H₂O: C, 64.46 (64.45); H, 6.46 (6.45); N, 9.70 (9.45).FAB-MS (+): m/z: 1119.7 [M+H]⁺.

(2) Preparation of TREN-1-Me-3,2-HOPO-TAM-O₂—NH₂ (27D)

Benzyl protected TREN-1-Me-3,2-HOPO-TAM-O₂—NH₂ (0.83 g, 0.74 mmol) wasdissolved in AcOH (10 ml) and concentrated HCl (10 ml) and stirred for 3days. The solvents were removed, and the residual solid was dissolved inMeOH (2 ml), H₂O (10 ml) and concentrated HCl (5 ml) for 10 h. After theremoval of the solvent, the residue was dissolved in MeOH (10 ml) andthe solvent again removed by rotary evaporation. This procedure wasrepeated six times, with 2 drops 6 M HCl being added to the first threesolutions. The final residue was dissolved in MeOH (3 ml) and added to arapidly stirring solution of Et₂O (150 ml) to afford a white powderwhich was filtered, and dried under vacuum. (Yield: 0.48 g, 73%). ¹H NMR(d₆-DMSO/D₂O, 500 MHz): δ=7.27 (d, 1H, TAM), 7.24 (d, 1H, TAM), 7.06 (d,J=7.4 Hz, 2H, HOPO), 6.35 (d, J=7.4 Hz, 2H, HOPO), 3.43-3.63 (m,aliphatic H) ppm. Anal. Calcd (Found) for C₃₄H₄₆N₈O₁₂.2HCl.3H₂O: C,46.11 (46.48); H, 6.14 (6.16); N, 12.65 (12.21). FAB-MS (+): m/z: 759[M+H]⁺.

Example 19 Preparation of the Gadolinium(III) Ion Complex withTREN-1-Me-3,2-HOPO-TAM-O₂—NH₂ (27E)

Imidazole (0.058 g, 0.85 mmol) was added to a solution of GdCl₃.6H₂O(0.060 g, 0.16 mmol) and TREN-HOPO-TAM-O₂—NH₂ (0.150 g, 0.169 mmol) inH₂O (10 ml). The resulting pale yellow solution was heated under refluxfor 2 h. The solvent was removed to yield a yellow residue which wassuspended in —PrOH, sonicated and filtered three times. The resultinglight brown solid was dried under vacuum. (Yield: 0.117 g, 73%). Anal.Calcd (Found) for Gd(TREN-HOPO-TAM-O₂—NH₂)(H₂O)₂ C₃₄H₄₇N₈O₁₄Gd: C, 43.03(42.87); H, 4.99 (4.61); N, 11.81 (11.63). FAB-MS (+): m/z: 913.2[M+H]⁺.

Example 20 Preparation of TREN-1-Me-3,2-HOPO-TAM₂ (27H) (1) Preparationof TAM-thiazolide-ethanolamine (27F)

A solution of ethanolamine (0.32 g, 5.26 mmol) in CH₂Cl₂ (100 ml) wasadded drop wise to a rapidly stirring solution of BnTAMdiThiaz (15.0 g,26.3 mmol) in CH₂Cl₂ (150 ml) under ambient conditions. The reaction wasstirred for 8 h, then the product was isolated by a gradient flashsilica column (2-10% CH₃OH in CH₂Cl₂). The solvents were removed toyield a yellow oil which solidified upon standing over several hours(Yield: 2.17 g, 79%). ¹H NMR (D₂O, 500 MHz): δ=8.17 (t, J=5.4 Hz, NH),7.93 (d, J=8.2 Hz, TAM CH), 7.39 (m, 10H, Bn H), 7.23 (d, J=8.2 Hz, 2H,TAM CH), 5.14 (s, 2H Bn CH₂), 5.13 (s, 2H Bn CH₂), 4.40 (t, J=7.3 Hz,2H, thiazolide CH₂), 3.62 (t, J=4.7 Hz, 2H, OCH₂), 3.39 (q, J=5.4 Hz,2H, NCH₂), 2.97 (t, J=7.3 Hz, thiazolide CH₂) ppm. Anal. Calcd (Found)for C₂₅H₂₆N₂O₅S₂: C, 62.05 (61.81); H, 5.01 (5.06); N, 5.36 (5.14) ppm.FAB-MS (+): m/z: 523[M+H]⁺.

(2) Preparation of Benzyl Protected TREN-1-Me-3,2-HOPO-TAM₂ (27G)

A solution of TAM-thiaz-ethanolamine (1.50 g, 2.81 mmol) in CH₂Cl₂ (100ml) was added dropwise to a rapidly stirring solution oftris(2-aminoethylamine) (TREN, 0.229 g, 1.57 mmol) in CH₂Cl₂ (100 ml).Small extra portions of 1-Bn-6-Me-3,2-HOPO-thiazolide were also added asdeemed necessary by TLC. The reaction mixture was purified by a gradientflash silica column (2-10% CH₃OH and 1% Et₃N in CH₂Cl₂). The solventswere then removed to yield a colorless oil. (Yield: 0.63 g, 34% relativeto TREN). ¹H NMR (d₆-DMSO, 400 MHz): δ=8.31 (br t, 2H, TAM NH), 8.22 (brt, 2H, TAM NH), 8.14 (t, 1H, HOPO NH), 7.45 (d, J=7.0 Hz, 1H, HOPO CH)7.29-7.40 (m, 29H, aromatic H), 6.21 (d, J=7.0 Hz, 1H, HOPO CH), 5.19(s, 2H, HOPO Bn CH₂), 5.03 (s, 8H, TAM Bn CH₂), 4.75 (t, 2H, OH), 3.45(s, 3H, HOPO CH₃), 3.24-3.33 (m, 20H, CH₂) ppm. Calcd (Found) forC₆₈H₇₁N₇O₁₃.2H₂O: C, 66.38 (66.60); H, 6.14 (6.20); N, 7.97 (7.83).FAB-MS (+): m/z: 1194.6 [M+1]⁺.

(3) Preparation of TREN-1-Me-3,2-HOPO-TAM₂ (27H)

Benzyl protected TREN-1-Me-3,2-HOPO-TAM₂ (0.470 g, 0.382 mmol) wasdissolved in AcOH (5 ml) and concentrated HCl solution (5 ml), and leftstirring for 3 days under ambient conditions. The solvents were removed,and the residue was dissolved in 6.0 M HCl and left stirring for another24 h (this was in order to hydrolyze acetyl ester that had been found tohave formed during the deprotection reaction). The solvents wereremoved, and the residue taken up in MeOH (5 ml), which was subsequentlyremoved by rotary evaporation (×3). The residue was then taken up inMeOH (3 ml) and added to rapidly stirring solution of Et₂O (200 ml), toyield a white precipitate which was filtered and dried under vacuum(Yield 0.248 g, 77%). ¹H NMR (d₆-DMSO, 300 MHz): δ=9.14 (br s, 2H, TAMNH), 8.93 (br s, 2H, TAM NH), 8.70 (br s, 1H, HOPO NH), 7.41 (d, J=8.8Hz, 2H, TAM CH), 7.34 (d, J=8.8 Hz, 2H, TAM CH), 7.13 (d, J=7.3 Hz, 1H,HOPO CH), 6.43 (d, J=7.3 Hz, 1H, HOPO CH), 3.36-3.75 (m, 23H, aliphaticH) ppm (Also evidence of approx 0.15 equiv. of NEt₃, presumably presentfrom the column in previous reaction). Calcd (Found) forC₃₃H₄₂N₇O₁₃Cl.0.15NEt₃HCl2.5H₂O: C, 48.14 (48.36); H, 5.89 (5.91); N,11.84 (11.84). FAB-MS (+): m/z: 744 [M+H]⁺.

Example 21 Preparation of TREN-Gly-1-Me-3,2-HOPO (6A) (1) Preparation ofGly-1-Me-3,2-HOPO (4A)

1-Me-HOPO-3,2-thiazolide (Formula 4, Scheme 1) (3.00 g, 8.334 mmol) wasdissolved in CH₂CH₂ (50 ml) and i-PrOH (50 ml) and added to a solutionof NaOH (0.350 g, 8.60 mmol) and glycine (0.626 g, 8.334 mmol) in water(3 ml). The yellow mixture was then stirred for 24 h, during which timeit turned colorless, and TLC indicated that the reaction was complete.The reaction mixture was purified by a gradient flash silica column(2-10% CH₃OH in CH₂Cl₂). The solvent was evaporated and the productpurified further by recrystallization from hot EtOH to afford whiteneedles (Yield: 2.06 g, 78%). ¹H NMR (CDCl₃, 300 MHz): δ=8.51 (t, J=5.4Hz, 1H, NH), 7.33-7.49 (m, 5H, Bn), 7.12 (d, J=7.2 Hz, 1H, HOPO CH),6.78 (d, J=7.2 Hz, 1H, HOPO CH), 5.43 (s, 2H, Bn CH₂), 4.05 (d, J=5.4Hz, 2H, glycine CH₂), 3.61 (s, 3H, N—CH₃) ppm. Anal. Calcd (Found) forC₁₆H₁₆N₂O₅: C, 60.75 (60.84); H, 5.10 (5.11); N, 8.86 (8.70). EI-MS (+):m/z: 317 [M +H]⁺.

(2) Preparation of Benzyl Protected TREN-Gly-1-Me-HOPO (5A)

N-hydroxysuccinimide (0.534 g, 4.64 mmol) was added to a solution ofGly-1-Me-HOPO (1.220 g, 3.87 mmol) in dry THF (60 ml) under a nitrogenatmosphere. After stirring for 20 min, dicyclohexylcarbodiimide (DCC,0.956, 4.64 mmol) and dimethylaminopyridine (DMAP, 0.044 g, 0.46 mmol)were added. After 9 h, a white precipitate of dicyclohexylurea (DCU) hadformed and the formation of the NHS-activated ester was judged to becomplete by TLC. TREN (0.141 g, 0.967 mmol, 0.25equiv.) was added. Thesolvent was evaporated and the residue taken up in a 1.0M aqueoussolution of HCl (50 ml). The suspension was filtered and the filtratewashed with CH₂Cl₂ (2×50 ml). The combined organic fractions were thenback-extracted once with 1.0 M HCl (50 ml). 10M NaOH was added drop-wiseto the combined aqueous fractions until the pH reached 11. The aqueousfractions were then extracted with CH₂Cl₂ (10×50 ml) and EtOAc (3×50ml). The organic extracts were combined, dried (Na₂SO₄) and evaporatedto dryness. The product was perified by a gradient flash silica column(2-8% CH₃OH in CH₂Cl₂). Evaporation of the solvent afforded a white foam(Yield: 0.753 g, 56%). ¹H NMR (CDCl₃, 400 MHz): δ=8.58 (t, J=5.4 Hz, 3H,HOPO-glycine NH), 7.27-7.51 (m, 18H, Bn and glycine-TREN NH), 7.00 (d,J=7.2 Hz, 3H, HOPO), 6.59 (d, J=7.2 Hz, 3H, HOPO), 5.42 (s, 6H, Bn CH₂),3.85 (d, J=5.4 Hz, 6H, glycine CH₂), 3.51 (s, 9H, N-CH₃), 3.20 (m, 6H,TREN NHCH ₂), 2.51 (m, 6H, TREN NCH₂) ppm. Anal. Calcd (Found) forC₅₄H₆₀N₁₀O₁₂.2H₂O: C, 60.21 (59.96); H, 5.99 (5.96); N, 13.00 (12.93).FAB-MS (+): m/z: 1041.5 [M+H]⁺.

(3) Preparation of TREN-Gly-1-Me-3,2-HOPO (6A)

TREN-Gly-1-Me-3,2-HOPO-Bn (0.650 g, 0.624 mmol) was dissolved in amixture of MeOH (325 ml) and EtOH (325 ml) and added to a slurry of 5%Pd on C (0.65 g) in EtOH (65 ml). The reaction was stirred under anatmosphere of hydrogen for 6 h. The solution was filtered and thesolvent removed by evaporation to afford a white solid (Yield: 0.296 g,62%). This solid was found to be hygroscopic, and was therefore storedin a vacuum desiccator. ¹H NMR (d₆-DMSO, 400 MHz): δ=8.71 (br s, 3H,HOPO-glycine NH), 7.95 (br s, 3H, glycine-TREN NH), 7.17 (d, J=7.3 Hz,3H, HOPO), 6.52 (d, J=7.3 Hz, 3H, HOPO), 3.92 (br s, 6H, glycine CH₂),3.46 (s, 9H, N-CH₃), 2.54 (br s, 6H, TREN CH₂) ppm. Anal. Calcd (Found)for C₃₃H₄₂N₁₀O₁₂.3.5H₂O: C, 47.54 (47.49); H, 5.92 (5.46); N, 16.80(16.48). FAB-MS (+): m/z: 771 [M+H]⁺.

Example 22 Preparation of the Gadolinium(III) Ion Complex withTREN-Gly-1-Me-3,2-HOPO (7A)

GdCl₃.6H₂O (0.0327 g, 0.124 mmol) was dissolved in a solution ofTREN-Gly-1-Me-3,2-HOPO (0.100 g, 0.138 mmol) in H₂O (30 ml). Then anexcess of aqueous ammonia solution was added to yield a light yellowprecipitate. The suspension was heated under reflux for 2 h. Aftercooling, the solution was reduced in volume and the product filtered anddried under vacuum. (Yield: 0.079 g, 71%). Anal. Calcd (Found) forGd(TREN-Gly-1-Me-3,2-HOPO)(H₂O)₂.3H₂O, C₃₃H₄₃N₁₀O₁₄Gd.3H₂O: N, 13.80(13.37); Gd, 15.49 (15.35). FAB-MS (+): m/z: 924 [M]⁺.

Example 23 Nuclear Magnetic Resonance Dispersion (NMRD) Study on Poly(Ethylene Glycol) Functionalized Gd(III) Complexes

The relaxivity parameters associated withGd-TREN-1-Me-3,2-HOPO-TAM-PEG-5000 were monitored by fitting the NuclearMagnetic Resonance Dispersion (NMRD) profile with a theoretical curvegenerated from a given set of relaxivity parameters. FIG. 16 below showsthe NMRD profile of Gd-TREN-1-Me-3,2-HOPO-TAM-PEG-5000 in water at 25°C. and pH=7.4. Solid curve fitted with r=3.1 Å, q=1, τ_(R)=300 ps,τ_(M)=31 ns (from ¹⁷O NMR), τ_(V)=24 ns, Δ²=7.3×10¹⁹, a=4.0 Å,D=2.24×10⁻⁵ cm²s⁻¹. The fit is relatively poor, which may reflect alarge degree of extra relaxivity due to the PEG chain organizing anetwork of hydrogen-bonded water molecules in the proximity of the metalcenter. This “second-sphere” relaxivity cannot be accounted forquantitatively and will most likely be reflected in the values for τ_(R)or the zero-field value of the electronic relaxation time, τ_(S0) beinglonger than in reality. Nevertheless, by far the best fit was obtainedwith q=1, in agreement with the value obtained from the ¹⁷O NMR study.In fact, the fit with q=2 (dotted line) is of very poor quality andforces τ_(R) to assume a value unreasonably low (147 ns) for a complexof this molecular size.

Example 24 70 NMR Study of Water Exchange on Poly (Ethylene Glycol)Functionalized Gd(III) Complexes

A variable temperature ¹⁷O NMR study of the transverse relaxation rateof H₂ ¹⁷O (R_(2p)) of Gd-TREN-1-Me-3,2-HOPO-TAM,Gd-TREN-1-Me-3,2-HOPO-TAM-PEG-2000 andGd-TREN-1-Me-3,2-HOPO-TAM-PEG-5000 at 2.1 T was carried out. Theprofiles of R_(2p) with temperature are shown in FIG. 17. Analysis ofthe profiles also allows the number of coordinated water molecules, q tobe evaluated. Interestingly, it was found that q=2 forGd-TREN-HOPO-1-Me-3,2-TAM, and q=1 forGd-TREN-1-Me-3,2-HOPO-TAM-PEG-2000 andGd-TREN-1-Me-3,2-HOPO-TAM-PEG-5000. The reduction in q that occurs inthe presence of a PEG chain can be explained by partial displacement thetwo water molecules by the PEG oxygen donors. Significantly thereappears to be an increase in τ_(M) as the PEG chain is lengthened, withvalues of 8±1, 19±2 ns and 31±2 ns for Gd-TREN-1-Me-3,2-HOPO-TAM andGd-TREN-1-Me-3,2-HOPO-TAM-PEG-2000, Gd-TREN-1-Me-3,2-HOPO-TAM-PEG-5000respectively.

The optimal value for τ_(M) depends on several variables, in particularthe field strength of the MRI scanner machine. Previous reports havesuggested optimal τ_(M) values of a few tens of nanoseconds. In order toinvestigate the likely optimal values of τ_(M) for the complexesreported herein, the optimal τ_(M) across a range of field strengthsthat are typical in MRI (FIG. 18) has been calculated. The resultsclearly show that the water exchange rates span a range which is optimalfor clinical MRI.

Example 25 Effect of Binding to HSA on the Relaxivity of Poly (ethyleneglycol) Functionalized Gd(III) Complexes

The relaxivity of Gd-TREN-1-Me-3,2-HOPO-TAM-PEG-5000 at pH=7.5 is 9.1mM⁻¹s⁻¹ (20 MHz, 25° C.), which compares to 8.8 mM⁻¹s⁻¹ (20 MHz, 25° C.)for Gd-TREN-HOPO-TAM. The increase in relaxivity observed upon additionof the PEG chain is very modest considering the large increase inmolecular weight, reflecting the decrease in q and the effect of rapidinternal motions of the PEG chain on τ_(R).

The longitudinal relaxation rate (R₁) of water protons in a 0.25 Msolution of Gd-TREN-1-Me-3,2-HOPO-TAM-PEG-5000 was measured withincreasing concentrations of HSA (FIG. 19) at 20 MHz and 25° C. Theresults clearly show an increase in R₁ as the HSA concentration isincreased. From these data, the relaxivity of theTREN-1-Me-3,2-HOPO-TAM-PEG-5000-HSA adduct was calculated to be 74±14mM⁻¹s⁻¹ with a formation constant, K_(a) of 186±50 M⁻¹. This representsrelatively weak binding, which would result in a mixture of bound andunbound complex under physiological concentrations of HSA. The value ofrelaxivity observed for this adduct is considerably higher than that ofany other complexes reported to date (per Gd^(III) center), reflectingan optimized water exchange rate and a slow rotational correlation time.

Example 26 Synthesis of 1,2-HOPOBn acid chloride (1) General

All chemicals were obtained from commercial suppliers (Aldrich orFisher) and were used as received. 6-Hydroxypicolinic acid was purchasedfrom Fluka. Me-3,2-HOPOBn-thiazolide was prepared as previouslydescribed (Xu et al., J. Med. Chem. 38: 2606-2614 (1995)). Reactionswere carried out under an atmosphere of nitrogen. Flash silica gelchomatography was performed using Merck 40-70 mesh silica gel. Unlessotherwise specified, all NMR spectra were recorded at ambienttemperature on a Bruker DRX 500, AMX 400 or AMX 300 spectrometer in theUniversity of California, Berkeley NMR laboratory. HPLC analyses wereperformed on a Varian Pro Star System with a Dynamax-60A C18-reversedphase column (mobile phase: 65% methanol in water). Microanalyses wereperformed by the Microanalytical Services Laboratory, College ofChemistry, University of California Berkeley. Mass spectra were recordedat the Mass Spectrometry Laboratory, College of Chemistry, University ofCalifornia, Berkeley.

(2) Synthesis of 6-carboxy-1,2-HOPO 35A

Acetic anhydride (100 mL) was mixed with 30% H₂O₂ solution (25 mL) withcooling. The mixture was stirred for 4 h until a homogenous peraceticacid solution formed. This peracetic acid solution was added slowly withstirring to a solution of 6-hydroxy-picolinic acid (Fluka, 25 g, 0.18mol) in a mixture of trifluoroacetic acid (150 mL) and glacial aceticacid (100 mL) (CAUTION! solid particles in the mixture cause vigorousoxygen evolution and can lead to an uncontrolled reaction). The mixturewas stirred at room temperature for 1 h, and then heated slowly to 80°C. (oil bath temperature) and kept at 80° C. for 10 h. A whiteprecipitate formed during this period, which was collected byfiltration, washed with cold methanol, and dried. It was dissolved inaqueous 10% KOH, heated to 80° C. for 6 hours, and re-precipitated withconcentrated HCl. The product was collected by filtration, washed withwater and dried in a vacuum oven. Yield 20.5 g (0.132 mol, 73%). mp176-177° C. Anal. Calc'd. (Found) for C₆H₅NO₄ (F.W. 155.15): C, 46.46(46.31); H, 3.25 (3.45), N, 9.03 (9.12). ¹H NMR (500 MHz, DMSO-d₆): δ6.63 (dd, J=7.0, 1.5 Hz, 1H), 6.71 (dd, J=9.2, 1.5 Hz, 1H), 7.43 (dd,J=9.0, 7.1 Hz, 1H). 13C NMR (125 MHz, DMSO-d6): δ106.9, 120.5, 134.9,137.3, 157.4, 163.3. IR (KBr pellet): 1734 cm⁻¹ (br, C═O); 1616 cm⁻¹ (m,C═O).

(3) Synthesis of 1-Benzyloxy-6-carboxy-2(1H)-pyridinone 36A

35A (15.5 g, 0.1 mol) and anhydrous potassium carbonate (27.6 g, 0.2mol) was mixed with benzyl chloride (15.2 g, 0.12 mol) in methanol (250mL). The mixture was refluxed for 16 h, filtered, and the filtrate wasevaporated to dryness. The residue was dissolved in water (50 mL) andacidified with 6 N HCl to pH 2. The resulting white precipitate wasisolated by filtration, washed with cold water, and dried in vacuum toyield 22.3 g (91%) of 36A, mp 176-177° C. Anal. Calc'd. (Found) forC₁₃H₁₁NO₄: C, 63.66 (63.75); H, 4.53 (4.55), N, 5.71 (5.52). ¹H NMR (500MHz, DMSO-d₆): δ 5.26 (s, 2H, CH2), 6.54 (dd, J=6.7, 1.1 Hz, 1 H), 6.73(dd, J=9.2, 1.6Hz, 1 H), 7.39-7.51 (m, 6 H). ¹³C NMR (125 MHz, DMSO-d₆):δ 77.9, 106.0, 124.1, 128.5, 129.1, 129.6, 133.8, 138.7, 140.4, 157.6,161.7.

(4) Synthesis of 1,2-HOPOBn acid chloride 37A

To a suspension of bezyloxyl,2-HOPO carboxylic acid (5.0 g, 20 mmol) intoluene or benzene (50-70 mL), excess oxalyl chloride (5.0 g) was addedwhile stirring. Gas was evolved and the suspension became clear upon theaddition of a drop of DMF as a catalyst. The mixture was then warmed to60° C. (oil bath) for 4-6 h. The solvent was removed by rotaryevaporation, leaving a pale brown oil. After co-evaporation twice withtoluene (5 mL), the residue was dissolved in dry THF, passed though aflash silica gel plug and eluted with dry THF. The 1,2-HOPOBn acidchloride so obtained after the solvent was removed under reducedpressure, was a thick, pale yellow oil: crude yield 5.0 g (95%). Thecrude product was used directly for reaction without furtherpurification. ¹H NMR (300 MHz, CDCl₃): δ5.32 (s, 2H, CH2), 6.88(d, J=7.0Hz, 1 H), 6.726(d, J=9.0 Hz, 1 H), 7.32-7.51(m, 6 H). ¹³C NMR (125 MHz,DMSO-d₆): δ 78.5, 112.2, 128.5, 128.6, 129.4, 130.3, 132.7, 136.4,140.1, 158.1, 158.8.

Example 27 Synthesis of 3,4,3-LI(1,2-Me-3,2-HOPO) (1) Synthesis of3,4,3-LI(1,2-HOPO)Bn 38A

To a solution of crude 1,2-HOPOBn acid chloride (5.0 g, 19 mmol) andtriethylamine (2.5 mL) in dry THF (60 mL), spermine (0.8 g, 4 mmol) wasadded in three portions while stirring. The mixture was heated at 60° C.(oil bath) overnight in a stoppered 100 mL round-bottomed flask. Thesolvent was then removed on a rotary evaporator, and the residue waspartitioned into a mixture of water (50 mL) and dichloromethane (50 mL).The organic phase was separated and it was washed successively with 1 MNaOH (100 mL), 1 M HCL (100 mL), and saline solution (100 mL), andloaded onto a flash silica column. Elution with 2-6% methanol indichloromethane allowed separation of the benzyl-protected precursor3,4,3-LI-(1,2-HOPOBn) as a white foam. Yield 70%. ¹H NMR (500 MHz,CDCl₃): δ 0.4-1.8(m, 16H), 2.8-3.6(m, 24H), 4.8-5.1(m, 2H), 4.88-5.05(m,2H), 5.15-5.30(m, 4H), 5.30-5.45(m, 2H), 6.00-6.46(m, 4H), 6.55-6.70(m,4H), 7.25-7.55(m, 24H), 8.72-8.95(m, 2H, NH). ¹³C NMR (125 MHz,DMSO-d₆): δ 23.4, 24.2, 24.5, 24.6, 26.4, 26.6, 26.7, 27.3, 27.7, 36.6,36.7, 41.8, 42.0, 42.4, 43.7, 46.2, 47.4, 47.7, 48.1, 79.2, 102.7,104.7, 123.1, 128.3, 128.4, 128.7, 129.2, 129.3, 129.4, 130.0, 130.1,130.2, 130.3, 130.4, 132.8, 132.9, 133.0, 133.1, 138.2, 142.0, 142.5,142.6, 143.3, 157.9, 158.1, 158.3, 160.4, 160.6, 161.2, 161.3. MS(FAB+):1111.5(MH+).

(2) Synthesis of 3,4,3-LI(1,2-HOPO) 39A

The precursor, 3,4,3-LI-(1,2-HOPOBn) was deprotected at room temperatureover four days by the action of 1:1 HCl(37%)/glacial HOAc. All of thevolatiles were removed in vacuo, and the resulting residue was dissolvedin a minimum amount of water, filtered and evaporated to dryness: yield81%. ¹H NMR (400 MHz, DMSO-d₆): δ 0.25-1.87(m, 8H), 2.81-3.63 (m, 24H),6.11-6.22 (m, 3H), 6.29-6.34(m, 2H), 6.48-6.58(m, 4H), 7.31-7.42(m, 4H),8.82(q, J=7.2 Hz,1H). 8.91(q, J=7.2 Hz,1H). ¹³C NMR (125 MHz, D₂O): δ23.1, 23.7, 24.2, 25.4, 26.7, 36.6, 36.8, 41.9, 44.0, 45.8, 47.7, 48.1,106.2, 106.5, 108.7, 109.0, 118.9, 120.1, 138.8, 139.0, 139.6, 140.0,140.5, 159.2, 161.1, 161.2, 162.2. MS(FAB+): 751(MH+). Anal. forC₃₄H₃₈N₈O₁₂.H2O.2HCl (841.68), Calc'd. (found): C, 48.52 (48.16); H,5.03 (4.82); N, 13.31 (13.23).

(3) Synthesis of 3,4,3-LI-Bis(Me-3,2-HOPOBn) 41A

To a solution of spermine (0.5 g, 2.5 mmol) in dry dichloromethane (60mL), Me-3,2-HOPO-thiazolide (2.0 g, 5.5 mmol) was added while stirring.The mixture was stirred at room temperature overnight, then washed with1 M KOH solution (30 mL×3). The organic phase was then dried in vacuum,leaving a pale brown oil: yield 85%. ¹H NMR(300 MHz, CDCl₃): δ 1.434(m,br, 4H), 1.506(quint, J=6.7 Hz, 4H), 2.443(t, J=6.7 Hz, 8H), 3.255(q,J=6.7 Hz, 4H), 3.575(s, 6H), 5.341(s, 4H, OCH2), 6.725(d, J=7.2 Hz, 2H),7.101(d, J=7.2 Hz, 2H), 7.2-7.5(m, 10H), 8.095(t, J=5.3 Hz, 2H). ¹³C NMR(125 MHz, CDCl₃): δ 27.25, 28.74, 37.43, 37.69, 46.84, 49.22, 74.56,104.66, 128.52, 128.60, 128.81, 130.72, 132.00, 136.12, 146.08, 159.42,163.16. MS(FAB+): 685.3(MH+).

(4) 3,4,3-LI(1,2-Me-3,2-HOPO)Bn 41B

The crude 3,4,3-LI-Bis(Me-3,2-HOPO Bn), 41A, (0.82 g, 1.2 mmol) wasdissolved in dry THF (50 mL) containing triethylamine (1.2 mL) andslowly added over 4 h to a solution of crude benzyloxy 1,2-HOPO acidchloride (1.7 g, 6.4 mmol) in dry THF (60 mL). The reaction mixture wasmaintained overnight at 60° C. After removing the solvents, the residuewas partitioned into a mixture of water (50 mL) and dichloromethane (50mL). The resulting organic phase was washed successively with 1 M NaOH(100 mL), 1 M HCl (100 mL), and saline water (100 mL), and loaded onto aflash silica column. Elution with 3-8% methanol in dichloromethaneallowed the separation of the benzyl-protected precursor as white foam:yield 75%. ¹H NMR(500 MHz, CDCl₃): δ0.9-1.6(m, 16H), 2.65-3.45(m, 24H),3.546(s, 6H), 4.90-5.02(m, 4H), 5.20-5.6(m, 8H), 5.80-6.10(m, 4H),6.54-6.80(m, 8H), 7.07-7.11(m, 4H), 7.15-7.51(m, 20H), 7.75 (m, br, 2H),7.99(m, br, 1H). ¹³C NMR (125 MHz, CDCl₃): δ 24.99, 26.35, 26.65, 27.66,36.47, 36.60, 36.68, 37.39, 41.80, 43.68, 45.61, 47.85, 74.39, 74.42,74.75, 74.82, 78.88, 79.03, 104.19, 104.22, 104.35, 104.40, 122.51,128.18, 128.20, 128.24, 128.41, 128.44, 128.47, 128.50, 128.54, 128.70,128.73, 128.75, 129.86, 129.91, 131.95, 133.11, 142.81, 157.80 157.93,159.14, 159.26, 161.42, 163.04, 163.12. MS(FAB+):1139.8.

(5) Synthesis of 3,4,3-LI(1,2-Me-3,2-HOPO) 43A

The benzyl-protected precursor 3,4,3-LI(1,2-Me-3,2-HOPO)Bn wasdeprotected in the same manner as 3,4,3-LI-(1,2-HOPO), above. The crudeproduct was dissolved in a minimum amount of distilled water. Pure3,4,3-LI(1 ,2-Me-3,2-HOPO) was precipitated as a beige solid uponcooling. The solid was filtered and dried under vacuum: yield 73% (basedon spermine). ¹H NMR(500 MHz, DMSO-d₆): δ 1.2-1.85(m, 16H), 2.85-3.35(m,24H), 3.453(s, 6H), 6.10-6.52(m, 6H), 7.10-7.41(m, 4H), 8.32(m, 1H), ¹³CNMR (500 MHz, D₂O/NaOD): δ 23.85, 24.02, 24.14, 24.19, 24.54, 24.83,25.05, 26.89, 27.06, 28.13, 28.30, 35.67, 35.71, 36.47, 36.51, 36.58,37.55, 42.93, 43.04, 43.16, 44.76, 44.90, 44.93, 45.11, 46.77, 48.34,48.41, 48.67, 105.74, 106.15, 106.22, 106.44, 106.93, 107.14, 114.87,114.96, 115.02, 115.87, 116.17, 119.61, 119.98, 132.70, 132.80, 133.06,142.87, 143.12, 160.37, 161.93, 162.21, 164.41, 164.47, 165.60, 165.75,169.49, 169.64. MS(FAB+):779.4(MH+), Anal. for C₃₆H₄₂N₈O₁₂.3H₂O.2HCl(905.76), Calc'd. (found): C, 47.74 (48.05); H, 5.56(5.22); N, 12.37(12.16).

The purity of compound 43A was confirmed by analytical HPLC. Thechromatogram indicates that the purity of the compound is above 99%.

Example 28 Synthesis of2,3-Dimethyl-5-benzyloxy-6-carboxy-4-pyrimidinone (1) Preparation oftetrahydropyran-2-yloxy-acetic acid ethyl ester (Formula 28, Scheme 4)

To a stirred solution of ethyl glycolate (35.3 g, 0.339 mol) containinga few crystals of p-toluene sulfonic acid, 3,4-dihydropyran (30.0 g,0.357 mol) was added dropwise (15 g over one hour followed by 15 g over30 min). After stirring overnight at room temperature, the mixture wasdiluted with diethyl ether (80 mL) and washed with a NaHCO₃ solution(from 30 mL sat. NaHCO₃ and 10 mL water). The organic layer wasseparated and dried (Na₂50₄) followed by evaporation of the ether. Theresidue was distilled under high vacuum to give 58.4 g (91.5%) of 28 asa clear liquid. ¹H NMR (CDCl₃, 400 MHz)δ: 1.29 (t, 3H, J=7.1 Hz, CH₃),1.53-1.95 (m, 6H, 3,4,5-THP-CH₂'s), 3.50-3.55 (m, 1H, 6-THP-CH₂),3.83-3.89 (m, 1H, 6-THP-CH₂), 4.19 (s, 2H, OCH₂CO₂R), 4.20 (t, 2H, J=7.1Hz, CH₂Me), 4.75 (m, 1H, THP-CH). ¹³C NMR (CDCl₃, 100 MHz) δ: 14.1,18.7, 25.2, 30.0, 60.7, 61.9, 63.8, 170.4.

(2) Preparation of2-Methyl-3H-5-tetrahydropyran-2-yloxy-6-tetrahydropyran-2-yloximethyl-4-pyrimidinone29

THPO-ethyl glycolate (34.0 g, 0.195 mol) in ether (180 mL) was stirredwith Na shot (2.24 g, 0.0974 mol) for 20 hrs under a N₂ atmosphere,resulting in a yellow solution. NaH (0.54 eq.) can be used in place ofNa, but usually requires the addition of 2-5% m/m EtOH to promote thereaction. The ether was removed and the residue covered with abs.ethanol (20 mL). An acetamidine solution was prepared fromacetamidine.HCl (9.32 g, 0.0989 mol), which was stirred for 2 hr insodium ethoxide in ethanol (130 mL, by addition of 2.36 g of Na). Thissuspension was filtered onto the ethanol covered residue from above andthe filter cake washed with ethanol (5 mL). The reaction mixture wasthen stirred and heated at reflux 3.5 hr, cooled to room temperature andthe solvent evaporated. The residue was dissolved in CH₂Cl₂ (80 mL) andHOAc was added (to pH 6, wet pH paper). After washing with water (2×80mL) the organic layer was dried (Na₂SO₄) and most of the solventremoved. To the viscous CH₂Cl₂ solution, hexanes (100 mL) was added,producing a white precipitate which was filtered and washed with hexanesto afford 2, 20.6 g (65.2%). Mp: 113-114° C. (+)FABMS: m/z 325 +. ¹H NMR(CDCl₃, 300 Mhz): δ 1.4-2.0 (m, 12H, 3,4,5-THP-CH₂), 2.42 (s, 3H,2-CH₃), 3.50-356 (m, 2H, 6-THP-CH₂), 3.86-3.96 (m, 2H, 6-THP-CH₂),4.44-4.81(4 d's, 2H, CH₂-OTHP), 4.79 (br m, 1H, THP-CH), 5.83 (br m, 1H,THP-CH), 11.44 (br s, 1H, NH). ¹³C NMR (CDCl₃, 100 MHz): δ 10.5, 18.5,18.5, 19.1, 19.3, 21.1, 25.0, 25.38, 25.40, 29.9, 30.3, 30.4, 61.9,62.2, 62.70, 62.74, 63.6, 63.8, 82.8, 98.4, 98.6, 98.63, 98.7, 138.58,138.63, 152.4, 152.9, 161.6. Anal. Calcd (found) for C₁₅H₂₄N₂O₅: C,59.24 (59.10); H, 7.46 (7.51); N, 8.64(8.94).

(3) Preparation of2,3-Dimethyl-5-tetrahydropyran-2-yloxy-6-tetrahydropyran-2-yloximethyl-4-pyrimidinone30

The THP-protected pyrimidine 29 (17.9 g, 54.9 mmol) dissolved in DMF (70mL) was dripped into a slurry of NaH (2.30 g, 60% in oil, 57.5 mmol) inDMF (100 mL) maintaining a gentle effervescence. The reaction wasstirred a for another 10 min then Mel (3.45 mL, 55.4 mmol) was added.After stirring 18 hr a few drops of methanol were added, followed aftera few minutes by evaporation of the solvent under reduced pressure. Theresidue was dissolved in CHCl₂ (150 mL) and washed with water (3×150mL). The solvent was removed and the residue dissolved in acetonitrile,then washed with hexanes (50 mL). After removal of the solvent thisafforded 16.2-18.6 g (87-100%) of the crude product (>90% pure by ¹HNMR). ¹H NMR (CDCl₃, 400 MHz): δ 1.52-1.89 (m, 12H, 3,4,5-THP-CH₂), 2.49(s, 3H, 2-CH₃), 3.49 (s, 3H, N-Me), 3.49-3.55 (m, 2H, 6-THP-CH₂),3.92-3.96 (m, 2H, 6-THP-CH₂), 4.39-4.82 (m, 3H, CH₂—OTHP+THP-CH), 5.77(br s, 1H, THP-CH).

(4) Preparation of2,3-Dimethyl-5-hydroxy-6-(hydroxymethyl)-4-pyrimidinone.HCl (31.HCl).

Method A. The protected pyrimidine 30 (3.38 g, 10 mmol) was dissolved in¹PrOH (20 mL), diethyl ether (20 mL) and conc. HCl (1 mL). Afterstanding at room temperature for 4 hr the crystallizing solution wasrefrigerated (0° C.) overnight. After filtration and washing with 1:1¹PrOH/Et₂O (3×5 mL) then Et₂O (10 mL), the white solid was dried underhigh vacuum at room temperature to give 1.83 g (88.8%) of 31.HCl.

Method B. A Hydrogen chloride/dioxane solution (50 mL, 4M) was drippedinto a solution of the protected pyrimidine 30 (30.1 g, 89 mmol) in EtOH(20 mL). A white solid rapidly precipitated and, after standing for afew hours at room temperature, the solid was filtered and washed withdioxane (3×10 mL) and diethyl ether (2×10 mL). Drying in vacuo overnightat 40° C. gave 16.08 g (88.8%) of 31HCl. This absorbs leq of H₂O uponstanding exposed in air. Mp:>200° C. (dec.). (+)FABMS: m/z 171 {MH]+. ¹HNMR (DMSO-d₆, 300 MHz): δ 2.68 (s, 3H, 2-Me), 3.49 (s, 3H, N-Me), 4.47(s, 2H, CH₂O). ¹³C NMR (DMSO-d₆, 100 MHz): δ 18.7, 31.8, 54.1, 132.9,138.0, 155.2, 157.2. Anal. Calcd (found) for C₇H₁₃ClN₂O₄: C, 37.43(37.65); H, 5.83 (5.91); N, 12.47 (12.46).

(5) Preparation of2,3-Dimethyl-5-benzyloxy-6-(hydroxymethyl)-4-pyrimidinone (32)

In a flask protected from direct light, DMF (300 mL), the abovepyrimidine, 31.HCl, (14.14 g, 68.43 mmol) and K₂CO₃ (20.0 g, 145 mmol)were mechanically stirred for 1 hr at 65° C., followed by addition ofBnCl (8.30 mL, 72.1 mmol) in one portion. Additional BnCl was added asthe reaction progressed if TLC indicated it necessary. After stirring 8hr at 65° C., the reaction mixture was cooled to room temperature,filtered (washing the cake with 2×20 mL DMF), and the solvent removed.The residue, in CH₂Cl₂ (100 mL), was filtered again and concentrated to˜20 mL at which point the cooled solution began to deposit a whitecrystalline mass. Dilution with diethyl ether (˜100 mL) affordedslightly off-white crystals (10.1 g, 56.7%) of 5. Mp: 93-95° C.(+)FABMS: m/z 261 [MH]⁺. ¹H NMR (CDCl₃, 300 MHz): δ 2.49 (s, 3H, 2-Me),3.19 (br s, 1H, OH), 3.53 (s, 3H, N-Me), 4.37 (s, 2H, CH₂O), 5.17 (s,2H, CH₂Ph), 7.28-7.40 (m, 5H, Ph). ¹³C NMR (CDCl₃ 100 MHz): δ 22.9,31.3, 58.8, 73.2, 128.3, 128.4, 128.6, 136.7, 137.5, 150.6, 153.9,158.9. Anal. Calcd (found) for C₁₄H₁₆N₂O₃: C, 64.60 (64.62); H, 6.20(6.04); N, 10.76 (10.82).

(6) Preparation of 2,3-Dimethyl-5-benzyloxy-6-carboxy-4-pyrimidinone(33)

The above pyrimidine, 32 (9.00 g, 34.6 mmol), TEMPO catalyst (55 mg),Adogen-464 phase transfer catalyst (690 mg) and NaBr (360 mg, 3.50 mmol)were combined in CH₂Cl₂ (400 mL) and water (10 mL). After cooling to 0°C. the reaction was stirred at 1500 r.p.m. while adding a cooled (10°C.) buffered bleach solution (125 mL commercial bleach+125 mL water+12.5 g NaHCO₃) keeping the reaction <4° C. (takes approx. 15 min).After a further 5 min, 2M NaOH solution was added until a solution of pH10 was obtained. The CH₂Cl₂ layer was separated and extracted with basicwater (pH 10, 100 mL). The combined aqueous solutions were washed withCH₂Cl₂ (50 mL). The aqueous phase was concentrated to ˜200-250 mL andcarefully acidified (conc. HCl) to pH 2, concomitant with precipitation.After standing at 5° C. overnight the white solid was filtered, washedwith water and dried under high vacuum to give 5.20 g (54.9%) of 33. Mp:180-181° C. (+)FABMS: m/z 275. 1H NMR (CDCl₃, 300 MHz): δ 2.53 (s, 3H,2-Me), 3.55 (s, 3H, N-Me), 5.46 (s, 2H, BnCH₂), 7.30-7.38 (m, 3H, Ph),7.49-7.52 (m, 2H, Ph). ¹³C NMR (DMSO-d₆, 100 MHz): δ 22.6, 31.2 73.3,128.0, 128.2, 128.3, 136.9, 139.1, 143.0, 155.7, 159.2, 165.7. Anal.Calcd (found) for C₁₄H₁₄N₂O₄: C, 61.31(61.36); H, 5.14 (5.06); N,10.21(10.30).

Example 29 Synthesis of2,3-Dimethyl-5-benzyloxy˜6-carboxy-4-pyrimidinone (33) via2,3-Dimethyl-5-benzyloxy-6-carboxy-4-pyrimidinone ethyl ester (35) (1)Preparation of 2-Methyl-3H-5-benzyloxy-6-carboxy-4-pyrimidinone ethylester 34

Benzyl benzyloxyacetate (14.13 g, 55.13 mmol) and diethyloxalate (8.060g, 55.13 mmol) and ethanol (0.2 mL) were stirred in dry THF (100 mL)with NaH (2.34 g, 60% in oil, 58.5 mmol) at room temperature for 24 hr.The THF was removed on a rotary evaporator and the residue dissolved inethanol (100 mL) followed by addition of sodium ethoxide (3.75 g, 55.1mmol) and acetamidine hydrochloride (5.21 g, 5.11 mmol). After stirringat 60° C. for 1.5 hr, the resulting suspension was cooled to roomtemperature and the solvent removed. The resulting oil was partitionedbetween CH₂Cl₂ (80 mL) and water (50 mL) and the pH adjusted to ˜6.Filtration was performed if necessary. The CH₂Cl₂ was separated andcombined with a CH₂Cl₂ (30 mL) wash of the aqueous phase. The CH₂Cl₂solution was washed with water, separated, dried (Na₂SO₄) and the volumereduced until a thick oil was obtained. This was immediately shaken withdiethyl ether (30 mL) and a white solid precipitated. After dilutionwith hexanes (˜10% by volume) the solution was filtered and the cakewashed with cold ether (3×10 mL). After drying, 7.2 g (45%) of 34 wasobtained as a pale powder. Mp: 125-126° C. (+)FABMS: m/z 289 ([MH]+). ¹HNMR (CDCl₃, 400 MHz): δ 1.31 (t, 3H, J=7.1 Hz, ethyl-Me), 2.50 (s, 3H,2-Me), 4.35 (q, 2H, J=7.1 Hz, OCH₂), 5.25 (s, 2, NCH₃), 7.32-7.38,7.44-7.46 (m, 3+2H, Ph), 13.14 (br t, 1H, amide NH). ¹³C NMR (CDCl₃, 100MHz): δ 14.1, 21.3, 62.2, 74.5, 128.4, 128.5, 136.4, 141.9, 144.9,153.6, 162.2, 164.2. Anal. Calcd (found) for C₁₅H₁₆N₂O₄: C, 62.49(62.51); H, 5.59 (5.62); N, 9.72 (9.71).

(2) Preparation of 2,3-Dimethyl-5-benzyloxy-6-carboxy-4-pyrimidinoneethyl ester 35

To a stirred suspension of 33 (0.380 g, 1.39 mmol) in CH₂Cl₂ (20 mL) wasadded carbonyl diimidazole (0.230 g, 1.42 mmol). The suspension rapidlydissolved and after ˜3 min the solution was diluted with ethanol (20 mL)and stirred overnight. After chromatography (SiO₂, 2% MeOH/CH₂Cl₂) andrecrystallization from ether/hexanes, 35, (0.19 g, 45%) was afforded asa white powder. Mp: 110-111.5° C. (+)FABMS: m/z 303 ([MH]+). ¹H NMR(CDCl₃, 400 MHz): δ 1.29 (t, 3H, J=7.1 Hz, ethyl-Me), 2.52 (s, 3H,2-Me), 3.55 (s, 3H, NCH₃), 4.32 (q, 2H, J=7.1 Hz, OCH₂), 5.23 (s, 2,BnCH₂), 7.30-7.36, 7.45-7.47 (m, 3+2H, Ph). ¹³C NMR (CDC¹³, 100 MHz): δ13.9, 23.0, 31.5, 61.8, 74.0, 128.1, 128.2, 128.4, 136.5, 141.2, 141.6,154.1, 159.8, 164.2. Anal. Calcd (found) for C₁₆H₁₈N₂O₄: C, 63.56(63.78); H, 6.00 (6.09); N, 9.27(9.24).

(3) Preparation of 2,3-Dimethyl-5-benzyloxy˜6-carboxy-4-pyrimidinone 33via 2,3-Dimethyl-5-benzyloxy˜6-carboxy-4-pyrimidinone ethyl ester 35.

To a stirred suspension of NaH (0.745 g, 60% in oil, 18.6 mmol) in dryDMF (30 mL) was added dropwise a solution of 34 (4.88 g, 16.9 mmol) inDMF (20 mL) over ˜20 min (maintaining gentle effervescence). After H₂evolution had ceased, methyl iodide (1.27 mL, 20.4 mmol) was added inone portion and the reaction stirred at ambient temperature. Finecrystals deposited and after ˜2 hr the reaction was complete (by TLC,silica gel, 4% MeOH/CH₂Cl₂). The excess hydride was quenched withethanol (2 mL) and the DMF removed by rotary evaporation. Addition ofwater produced an oily solid which became an off-white crystalline massupon further shaking. This solid was separated by filtration, driedbriefly and then washed with hexanes (3×20 mL) to afford 3.0 g (˜10mmol) of crude 35 (>95% pure by NMR). The spectroscopic properties ofthis material corresponded to those of 35 synthesized in Preparation 1.This crude product was dissolved in methanol (50 mL) and KOH (0.660 g,11.8 mmol) and stirred at room temperature. After 6 hr the hydrolysiswas complete and a fine white precipitate had formed. After evaporationof the solvent under reduced pressure the residue was dissolved in water(20 mL), filtered and slowly acidified with conc. HCl. A small amount ofyellowish sticky solid initially precipitated and the solution wasdecanted from this. Acidification continued (to pH 2) and a white solidwas isolated by filtration, followed by an aqueous wash (2×10 mL) togive after drying in vacuo 2.63 g of 33 (57% from 34). This material hadidentical spectroscopic properties to 33 synthesized in Preparation 1.Mp. 179-180° C. Anal. Calcd (found) for C₁₄H₁₄N₂O₄: C, 61.31 (61.50); H,5.14 (5.13); N, 10.21 (10.30).

Example 30 Ligand Syntheses from HOPY Acid (33) (1) Preparation ofTris[(2,3-Dimethyl-5-benzyloxy-6-carboxamido-4-pyrimidinone) ethyl]amine33A.

To a slurry of HOPY acid 33 (1.99 g, 7.26 mmol) in CH₂Cl₂ (20 mL),carbonyl diimidazole (1.25 g, 7.71 mmol) was added in 0.3 g portionsover 10 min. After a further 5 min tris(2-aminoethyl)amine (0.350 g,2.39 mmol) in CH₂Cl₂ (5 mL) was added dropwise over 5 min and thereaction stirred overnight. The CH₂Cl₂ was removed and the resultant oilshaken into ethanol (20 mL). Upon standing, a white crystalline massseparated which was filtered and washed with ethanol (5×5 mL). Afterdrying under high vacuum at 40° C., 1.53 g (69%) of (33A) was isolated.Mp: 110-113° C. (+)FABMS: m/z 915 [MH]+. ¹H NMR (CDCl₃, 400 MHz): δ 2.28(s, 3H, 2-Me), 2.64 (t, 2H, J=5.8 Hz, cap NCH₂), 3.34-3.37 (m, 3+2H,NCH₃+CH₂NHCOR), 5.10 (s, 2H, BnCH₂), 7.27-7.35, (m, 3H, Ph), 7.50-7.52(m, 2H, Ph), 7.79 (br t, 1H, amide NH). ¹³C NMR (CDCl₃, 100 MHz): δ22.8, 31.9, 37.8, 53.3, 74.6, 128.3, 128.4, 129.0, 136.7, 141.1, 141.4,153.4, 160.1, 162.9. Anal. Calcd (found) for C₄₈H₅₄N₁₀O₉.H₂O: C, 61.79(61.48); H, 6.05 (5.98); N, 15.01(14.98).

(2) Preparation ofN-Ethyl-2,3-dimethyl-5-benzyloxy-6-carboxamido-4-pyrimidinone 33B.

Synthesized by the method above for 33A, and isolated as a white solidin 59% yield after chromatography (SiO₂, 2% MeOH/CH₂Cl₂). (+)FABMS: m/z302 [MH]⁺. ¹H NMR (CDCl₃, 400 MHz): δ 1.08 (t, 3H, J=7.3Hz, ethyl-Me),2.49 (s, 3H, 2-Me), 3.33 (m, 2H, CH₂NHCOR), 3.50 (s, 3H, NMe), 5.22 (s,2H, BnCH₂), 7.27-7.35, 7.38-7.40 (m, 3+2H, Ph), 7.41 (br s, 1H, amideNH). ¹³C NMR (CDCl₃, 100 MHz): δ 14.4, 23.1, 31.6, 34.3, 74.7, 128.4,128.9, 136.3, 140.6, 141.9, 153.1, 160.3, 162.4. Anal. Calcd (found) forC₁₆H₁₉N₃O₃: C, 63.77 (63.84); H, 6.36 (6.27); N, 13.94 (14.00).

(3) Preparation ofN,N-Dimethyl-2,3-Dimethyl-5-benzyloxy-6-carboxamido-4-pyrimidinone 33C

Synthesized by the method above for 33A and isolated as a white solid in68% yield after chromatography (Si0₂, 2% MeOH/CH₂Cl₂). (+)FABMS: m/z 302[MH]⁺. ¹H NMR (CDCl₃, 400 MHz): δ 2.48 (s, 3H, 2-Me), 2.74 (s, 3H, amideN-Me), 2.99 (s, 3H, amide N-Me), 3.52 (s, 3H, NMe), 5.16 (s, 2H, BnCH₂),7.26-7.33, 7.38-7.40 (m, 3+2H, Ph). ¹³C NMR (CDCl₃, 100 MHz): δ 23.04,31.43, 34.30, 37.50, 74.02, 128.11, 128.27, 128.43, 136.83, 138.43,145.98, 154.95, 159.50, 165.56. Anal. Calcd (found) for C₁₆H₁₉N₃O₃: C,63.77 (64.08); H, 6.36 (6.35); N, 13.94 (13.90).

(4) Preparation ofTris[(2,3-Dimethyl-5-hydroxy-6-carboxamido-4-pyrimidinone)ethyl]amine33D

The benzyloxypyrimidinone 33A (1.12 g, 1.22 mmol) was dissolved inacetic acid (10 mL) and 5%Pd/C (60 mg) was added. The reaction mixturewas stirred under an atmosphere of H₂ for 2 hrs at room temperature. ThePd/C was removed by filtration and the solution concentrated to a thickoil. Dilution with MeOH (5 mL) then water (25 mL) lead to precipitationof the product as a white powder which was dried in vacuo to afford 0.70g of 33D (86%). Mp: 217-219° C. (melts and resolidifies), 242-244° C.(dec.). (+)FABMS: m/z 665 [MH]⁺. ¹H NMR (CDCl₃, 400 MHz): δ 1.63 (br s,2H, water), 2.36 (br s, 3H, 2-Me), 2.81 (br t, 2H, J=6 Hz, cap NCH₂),3.48 (br t+s, 2+3H, CH₂NHCOR+NCH₃), 7.83 (br, t, 1H amide NH), 11.91 (s,1H, OH). ¹³C NMR (CDCl₃, 100 MHz): δ 22.2, 31.3, 37.1, 52.6, 125.1,145.4, 149.0, 157.5, 168.1. Anal. Calcd (found) for C₂₇H₃₆N₁₀O₉.H₂O: C,48.94 (48.69); H, 5.78 (5.89); N, 21.14 (20.78).

(5) Preparation ofN-Ethyl-2,3-dimethyl-5-hydroxy-6-carboxamido-4-pyrimidinone 33E

Synthesized by the method above for 33D as a white solid in 95% yieldafter crystallization of the crude filtrate from 2-propanol/ether. Mp:138-139° C. (+)FABMS: m/z 212[MH]⁺. ¹H NMR (CDCl₃, 400 MHz): δ 1.24 (t,3H, J=7.3 Hz, ethyl-Me), 2.44 (s, 3H, 2-CH₃), 3.43 (m, 2H, CH₂NCOR),3.52 (s, 3H, NCH₃), 7.61 (br s, 1H, amide NH), 12.06 (s, 1H, OH). ¹³CNMR (CDCl₃, 100 MHz): δ 14.5, 22.5, 31.6, 33.9, 125.2, 146.4, 147.9,158.6, 168.0. Anal. Calcd (found) for C₉H₁₃N₃O₃: C, 51.18 (51.40); H,6.20 (6.10); N, 19.89 (19.79).

(6) Preparation ofN,N-Dimethyl-2,3-dimethyl-5-hydroxy-6-carboxamido-4-pyrimidinone 33F

Synthesized by the method above for 33D as a white solid in 92% yieldafter crystallization of the crude filtrate from ethanol. Mp: 203-204°C. (+)FABMS: m/z 212 [MH]⁺. ¹H NMR (CDCl₃, 400 MHz): δ 2.42 (s, 3H,2-Me), 3.04 (br s, 3H, amide NCH₃), 3.22 (br s, 3H, amide NCH₃), 3.50(s, 3H, NCH₃), 9.98 (s, 1H, OH). ¹³C NMR (CDCl₃, 100 MHz): δ 22.5, 31.6,36.2, 38.7, 130.4, 143.2, 147.5, 159.2, 167.2. Anal. Calcd (found) forC₉H₁₃N₃O₃: C, 51.18 (51.10); H, 6.20 (6.08); N, 19.89 (19.88).

Example 31 Synthesis of Ligands with PEG Substituents (1) Preparation ofTREN-bis(6-Me-1-Bn-HOPOBn₂)-NH₂ (Formula 51, Scheme 10)

A solution of 6-Me-1-Bn-3,2-HOPO-thiaz (1.125 g, 2.50 mmol) in CH₂Cl₂(100 ml) was added dropwise, over 20 h, to a rapidly stirring solutionof tris(2-aminoethylamine) (TREN, 0.201 g, 1.37 mmol) in CH₂Cl₂ (100ml). The reaction was allowed to continue for 30 h (TLC indicated theformation of the tris-, bis-, and mono-substituted products). Thesolvent was evaporated and the bis-substituted product purified by flashcolumn chromatography (silica, eluent: 99:1 CH₂Cl₂:NEt₃ with increasinggradient of MeOH to 5%). Evaporation of the solvents yielded a whitefoam (Yield: 1.02 g, 92.0%). ¹H NMR (CDCl₃, 400 MHz): δ=8.02-8.08 (t,2H, amide NH), 7.02-7.36 (m, 20H, ar. H), 6.52 (s, 1H, HOPO), 6.48 (s,1H, HOPO), 5.33 (s, 4H, Bn-CH₂), 5.29 (s, 4H, Bn-CH₂), 3.26 (m, 4H, TRENHN-CH ₂), 2.32-2.47 (m, 8H, TREN CH₂), 2.18 (s, 3H, CH₃), 2.19 (s, 3H,CH₃).

(2) Preparation of PEG550-Cl (52A)

SOCl₂ (2.32 mL, 32 mmol) was added to a solution of poly(ethyleneglycol) methyl ether (avg. M_(n) ca. 550) (10.89 ml, 18 mmol) in toluene(20 ml) at 70° C. The temperature was reduced to room temperature after2 days and stirring continued for another 2 days. The reaction mixturewas then neutralized to pH 7. The solvent was evaporated and then thecrude product dissolved in CH₂Cl₂. The un-dissolved salt was filteredand the organic layer was washed with water (80 ml×2). The CH₂Cl₂ wasevaporated to yield a light yellow semi-solid (yield: 10.4 g, 98.0%).FAB-MS(+), m/z 579 (median) [M+1]⁺ with 12 (CH₂CH₂O) units. The peaksare separated by 44 m/z units in the range 359-887 m/z corresponding to7-19 (CH₂CH₂O) units.

(3) Preparation of PEG550-phthalimide (52B)

PEG550-Cl (10.40 g, 18.0 mmol) and potassium phthalimide (10.74 g, 58.0mmol) were dissolved in dry DMF (75 ml) and heated at 130° C. for 42 h.The reaction mixture was filtered and the filtrate evaporated. Theresulting yellow paste was partitioned between 1:1 CH₂Cl₂:H₂O (100ml×4), the organic component extracted and dried in MgSO₄. Evaporationof CH₂Cl₂ yielded crude product which was purified twice by flash columnchromatography (silica, eluent: CH₂Cl₂ with increasing gradient of MeOHfrom 0-10%). The product was isolated as a yellow oil (yield: 6.83 g,55%). ¹H NMR (CDCl₃, 400 MHz): δ=7.61-7.71 (m, 2H, ar. H), 7.51-7.57 (m,2H, ar. H), 3.69 (t, 2H, CH₃O—CH ₂), 3.54 (t, 2H, CH₃OCH₂—CH ₂),3.33-3.50 (m, large integral, PEG-CH₂), 3.17 (s, 3H, CH₃) ppm.

(4) Preparation of PEG550-NH₃.Cl (52C)

PEG550-phthalimide (16, 5.19 g, 7.53 mmol) and hydrazine monohydrate(1.26 g, 25.2 mmol) were refluxed for 2 h in MeOH (120 ml). The reactionmixture was cooled and the solvent evaporated resulting in a white solidresidue. H₂O (200 ml) and concentrated HCl (10 ml) were added and themixture refluxed for 1 h then cooled to 0° C. The phthalhydrazine whiteresidue was removed by filtration and the filtrate evaporated yieldingthe product as a semi-solid. This semi-solid was washed with anhydrousethanol and the resulting precipitate filtered. This step was repeatedwith MeOH and the filtrate evaporated to yield a semi-solid which wasdried under vacuum (yield: 4.19 g, 99%). ¹H NMR (D₂O, 400 MHz):δ=3.54-3.72 (m, 48H, CH₂), 3.31 (s, 3H, CH₃) ppm. FAB-MS (+), m/z: 516(median); [MH]⁺with 11 (CH₂CH₂O) units. The peaks are separated by 44m/z units in the range 296-693 m/z, corresponding to 6-15 (CH₂CH₂O)units.

(5) Preparation of PEG550-TAM-thiaz (Formula 52, Scheme 10)

PEG550-NH₃.Cl (0.54 g, 98 mmol) in CH₂Cl₂ (200 ml) was added dropwise toa rapidly stirring solution of 23 (5.73 g, 987 mmol) in dry CH₂Cl₂ (250ml) over the course of 48 hours. Triethylamine (0.09 g, 100 mmol) inCH₂Cl₂ (50 ml) was added dropwise to the yellow solution overnight. Thesolvent was then evaporated and the product purified by columnchromatography (silica, eluent: CH₂Cl₂, with increasing gradient of MeOHfrom 0-9%). The product was isolated as a yellow oil (0.806 g, yield87.0%) ¹H NMR (CDCl₃ 400 MHz) δ=8.05 (s, 1H, NH), 7.89 (d, 1H, TAM H),7.18-7.7 (m, 10H, Bn H), 7.19 (d, 1H, TAM H), 5.09 (s, 4H, Bn CH₂), 4.35(t, 2H, thiaz CH₂), 3.46-3.64 (m, 44H, PEG CH₂), 3.36 (s, 3H, PEG CH₃),2.90 (t, 2H, thiaz CH₂) ppm. FAB-MS(+), m/z 977 (medium) [M+1]⁺ with 11(CH₂CH₂O) units. The peaks are separated by 44 m/z units in the range768-1153 m/z corresponding to 6-15 (CH₂CH₂O) units.

(6) Preparation of TREN-bis(6-Me-HOPO-Bn₂)-(Bn₂-TAM-PEG550) (Formula 53,Scheme 10)

51 (0.72 g, 0.89 mmol) and 52 (0.72 g, 0.73 mmol) were stirred in CH₂Cl₂(40 ml) under N₂ for two days. The solution remained bright yellow andTLC indicated the reaction was incomplete. Triethylamine (0.09 g, 0.89mmol) and dimethyl-aminopyridine (DMAP, 0.01 g, 0.09 mmol) were addedand the reaction continued for two days. The yellow solution was thenpartitioned between CH₂Cl₂ and 0.5 M KOH solution. The organic componentwas extracted, dried with MgSO₄, and then filtered. The solvent wasevaporated and the brown oil residue purified by flash columnchromatography (silica: eluent: CH₂Cl₂ with increasing gradient of MeOHfrom 0-5%). The product was isolated as a light brown oil (yield: 1.08g, 88%). ¹H NMR (CDCl₃, 400 MHz): δ=8.06 (t, 1H, NH), 7.92 (t, 2H, NH),7.80 (d, 1H, TAM CH), 7.73 (d, 1H, TAM CH), 7.62 (t, 1H, NH), 7.26-7.39(m, 26H, aromatic H), 7.07 (m, 4H, aromatic H), 6.52 (s, 2H, HOPO H),5.31 (s, 8H, Bn CH₂), 5.14 (s, 2H, TAM Bn CH₂), 5.08 (s, 2H, TAM BnCH₂), 3.47-3.64 (m, 52H, PEG CH₂), 3.36 (s, 3H, PEG CH₃), 3.16 (m, 6H,TREN CH₂), 2.33 (m, 6H, TREN CH₂), 2.23 (s, 6H, HOPO CH₃) ppm.

(7) Preparation of TREN-bis(HOPO-Bn)-(TAM-PEG550) (Formula 54, Scheme10)

TREN-bis(HOPO-Bn₂)-(Bn₂-TAM-PEG550) (53, 0.36 g, 0.22 mmol) wasdissolved in acetic acid (10 ml) and then 12N HCl (10 ml) added to thesolution. The solution was stirred in the dark at room temperature for 2days. The solvents were evaporated yielding a yellow crystalline solid.MeOH (10 ml) was added and the solvent evaporated (×3). MeOH (2 ml) wasthen added to the residue and the resulting solution added dropwise to arapidly stirring solution of diethyl ether (450 ml). After stirringovernight, the mixture was filtered and the residue dried in vacuumovernight. The final product was isolated as a black, glass-like solid(yield: 0.29 g, 96%). ¹H NMR (d₆-DMSO, 400 MHz): δ=9.2 (br s, 1 H, NH),8.9 (br s, 1H, NH), 8.7 (br s, 2H, NH), 7.2-7.4 (m, 8H, Bn and TAM CH),7.06 (d, 4H, Bn CH), 6.38 (s, 2H, HOPO H), 5.27 (s, 4H, Bn CH₂), 3.73(m, 6H, TREN CH₂), 3.3-3.6 (m, large integral obscured by H₂O peak, PEGand TREN CH₂), 2.12 (s, 6H, HOPO CH₃) ppm. Anal. Calc. for 54.HCl.2H₂O(average of 11 ethers), (Found): C, 56.62 (56.81); H, 7.02 (7.07); N,7.11 (6.85). FAB-MS (+), m/z: 1307 (median); [MH]⁺with 11 (CH₂CH₂O)units. The peaks are separated by 44 m/z units in the range 1130-1483m/z, corresponding to 7-15 (CH₂CH₂O) units.

(8) Preparation of Gd-TREN-bis(HOPO-Bn)-(TAM-PEG550) (Formula 55, Scheme10)

TREN-bis(HOPO-Bn)-(TAM-PEG550) (54, 0.180 g, 0.131 mmol) andGd(acac)₃.2H₂O (0.060 g, 0.126 mmol) were dissolved in MeOH (15 ml) andheated under reflux for 15 minutes. Pyridine (0.030 g, 0.382 mmol) wasadded and heating continued for 2 h. The solvent was evaporated and theresidue purified by column chromatography (Sephadex LH-20, eluent:MeOH). The product isolated was then dissolved in MeOH (4 ml) and addeddropwise to a rapidly stirring solution of diethyl ether (450 ml). Themixture was stirred overnight and then filtered. The residue was driedunder vacuum overnight and isolated as a grey powder (yield: 0.135 g,73%). Anal. Calc. for H55.3H₂O (average of 11 ethers), (Found): C, 51.54(51.47); H, 6.25 (6.42); N, 6.47 (6.39). ES-MS (−), m/z: 1458 (median);[GdL]⁻, i.e. L with 11 (CH₂CH₂O) units. The isotopic abundance of theclusters are characteristic of mononuclear Gd-containing species. Thepeaks are separated by 44 m/z units in the range 1284-1636 m/z,corresponding to 7-15 (CH₂CH₂O) units.

Results

(1) Water Proton Relaxation

Table 3 shows some of the relaxivity properties of selected Gd complexesof the TREN-bisHOPO-TAM series.

TABLE 3 Gd-DTPA 55 58 27E 19 48 47 r_(1p)/mM⁻¹s⁻¹ 4.3 8.8 9.2 9.9 10.48.9 9.1 (20 MHz) K_(A) (HSA)/M⁻¹ <100 4242 ± 730 1823 ± 400 6860 ± 15002500 ± 400 959 ± 190 186 ± 50 r_(1p) ^(b)/mM⁻¹s⁻¹ 4.3 21 ± 1 16 ± 1 15.5± 0.3  22.5 ± 0.3 18 ± 1   74 ± 14 (HSA) τ_(M)/ns 303 11 8 / / 10 31Variable Temperature ¹⁷O NMR

The value of τ_(m), was independently obtained by a variable temperature(VT) ¹⁷O NMR measurement of the water nuclear transverse relaxation rate(R₂). The VT ¹⁷O NMR for 55 and 58 are shown in FIG. 20. The data weremeasured at 2.1 T (90 MHz for the proton and 12 MHz for 170) and pH≈7.The curve was analyzed in terms of the well established equations ofSwift-Connick, rearranged in a form suitable for Gd. As an initialestimate of the values of the structural and dynamic parameters, thosepreviously found for 47 were used in fitting the profile of 55. Anexcellent fit of the data was obtained with q=1, τ_(m)=11 ns, ΔH_(M)=20kJ/mol , Δ²=1.4×10²⁰ s⁻¹, τ_(V)=24 ps. For Gd^(III), the electronicrelaxation rate is usually ascribed to a transient zero field splitting(ZFS) brought about by solvent collisions or molecular vibrations. τ_(V)is a correlation time for the modulation of this ZFS and Δ² is the meansquare of the ZFS splitting energy.

Binding to HSA

An objective of synthesizing the Bn and MOB chelates was to investigatetheir interaction with HSA. The non-covalent interaction between HSA andthe Gd complexes was investigated using the well-established protonrelaxation enhancement (PRE) method that allows both the bindingparameters (KA) and the relaxivity enhancement of the (Gd³⁺ complex)-HSAadduct to be determined. In this method, the water proton longitudinalrelaxation rates (R₁) of solutions containing the Gd complex andincreasing concentrations of the serum protein are measured. The resultsof the PRE study of 55, 58 and 48 with HSA are shown in FIG. 21.

Example 32 Biodistribution Studies

A preliminary 24 hour study of the biodistribution of¹⁵³Gd[TREN-bisHOPO-(TAM_DME)] indicated that the complex is completelycleared from mice within 24 hours of administration. Approximately 75%of the injected dose is cleared via the liver and kidneys (excreted asurine and feces). The results for the percent-injected dose per gram oftissue are shown in FIG. 22.

Example 33 Solution Thermodynamic General Methods (1) General Methods

All solutions were prepared using distilled water that was furtherpurified by passing through a Millipore Milli-Q cartridge system(resistivity =18 MΩcm) and then degassed by boiling for at least 30 min.while bubbling with argon. Once prepared, solutions were protected fromthe ingress of oxygen and carbon dioxide by storing under a slightpositive pressure of argon, which was purified by passing through anAscarite II (A. H. Thomas) scrubber.

A solution of 0.100 M KCl was prepared from 99.99% KCl (FisherScientific) and was used to maintain constant ionic strength during alltitrations. Carbonate-free 0.1 M KOH was prepared from Baker Dilut-Itanalytic concentrated KOH and was standardized against potassiumhydrogen phthalate to a phenolphthalein endpoint. Gadolinium(III) andzinc(II) solutions, each ˜0.100 M in metal ion, dissolved in ˜0.100 MHCl were prepared from anhydrous 99.99% chloride salts (Alpha). Themetal ion content was checked by EDTA titration with Xylenol Orange asindicator using sodium acetate buffer. The proton concentration of thestandard solutions was checked by titration of a known volume of metalion solution and a slight excess of EDTA (˜1.005 eq.) to the equivalencepoint. (Harris et al., J. Am. Chem. Soc., 101:2722 (1979)). For alltitrations, the observed pH was measured as −log[H⁺]. The glasselectrode was calibrated in hydrogen ion concentration units bytitrating 2.000 mL of standardized HCl diluted in 50.00 mL of 0.100 MKCl, with 4.200 mL of standardized KOH. The calibration titration datawere analyzed by a nonlinear least-squares program. (Martell, A. E.;Motekaitis, R. M, Determination and Use of Stability Constants; VCH: NewYork (1988)).

(2) Potentiometric pH Titrations

As previously reported, (Turowski et al., Inorg. Chem., 27:474 (1988))potentiometric titrations were performed using an automated apparatusconsisting of a Accumet pH meter (models 925, 825MP or 15), a pHelectrode (Orion Ross semi-micro combination, Cole Parmer semi-microcombination or Corning high performance combination electrodes), anautoburet (Metrohm 665 Dosimat or 702 SM Titrino) fitted with a 5 mLpiston exchange unit and a jacketed Ar swept titration cell maintainedat 25.0° C. by a Lauda K-2/R or Neslab RTE-111 constant temperaturecirculating bath. The electronic systems were integrated for automatedcollection with an IBM PC clone.

In this study, ligand and metal complex solutions were titrated from lowto high pH and back again if possible. Titrations for the Zn/HOPYsystems were not reversible, presumably due to the formation of mixed MLhydroxide complexes. In this case, titrations from low to high pH werecarried out with differing point-by-point equilibration times (˜45-120see) to check for consistency in the determination. Formation constantscalculated from the potentiometric titration data were determined withthe aid of a FORTRAN non-linear least-squares refinement program (BETA90). (Harris et al., Am. Chem. Soc., 103:2667 (1981); Kappel et al.,Inorg. Chem., 21:3437 (1982)). Due to low solubility of the neutral LH₃species of Tren-Me₂-5,4-HOPY, titrations could not be carried out atligand concentrations of >0.25 mM. Although this is a low concentrationfor potentiometric titrations, the buffer regions corresponding to theprotonation steps are around neutral pH, and so could still bedetermined.

(3) Spectrophotometric pH Titrations

As previously reported, (Garrett et al., Am. Chem. Soc., 113:2965(1991)) spectrophotometric titrations were carried out in a custom-builtautomatic titration apparatus using a HP 8450A or HP 8452Aspectrophotometer and the pH monitoring equipment mentioned above forpotentiometric titrations. Solutions were titrated from low to high andhigh to low pH to ensure equilibrium had been achieved. At least threedata sets were collected and the spectra (˜50-100), pH values andvolumes were transferred to an IBM PC clone for analysis. Data from230-400 nm were used in the refinement. Models used to fit the titrationdata and determine formation constants were refined using the factoranalysis and least-squares refinement program REFSPEC. (Turowski et al.,Inorg. Chem., 27:474 (1988)).

(4) Relaxivity Measurements

Water proton relaxation measurements were carried out at 20 MHz with aStelar Spinmaster Spectrometer (Mede, Pv, Italy) on 0.5-2 mM solutionsof the Gd(III) complex. Spin-lattice relaxation times T₁ were measuredby the standard inversion recovery method with typical 90° pulse widthof 3.5 ms, 16 experiments of 4 scans. The reproducibility of the data is±1%. The temperature was controlled by a Stelar VTC-91 air-flow heaterequipped with copper-constantan thermocouple (uncertainty ±0.1° C). The1/T₁ nuclear magnetic relaxation dispersion (NMRD) profiles of waterprotons were measured from 0.00024 to 1.2 T (corresponding to the range0.01-50 MHz of proton Larmor frequencies) at 15, 25 and 39° C. using 1.5mM solutions of the complex on the field-cycling Koenig-Brownrelaxometer of the University of Torino (Italy). The temperature wascontrolled by circulating freon from an external bath and measured by athermometer inserted into the freon close to the sample. Thereproducibilities of the measured T₁ values were estimated to be ±2%.Technical details of the instrument and of the data acquisitionprocedure are given elsewhere. (Koenig, S. H.; Brown III, R. D., NMRSpectroscopy of Cells and Organism; CRC Press: Boca Raton (1987)). Thesample for the NMRD profile in blood serum was prepared by dissolving a1 mol L⁻¹ solution of the Gd(III) complex in a lyophilized serum ofhuman origin (Seronorm™, Nycomed) from controlled voluntary blood donorsof Scandinavian blood banks.

Variable-temperature ¹⁷O NMR measurements were recorded on a JEOL EX-400(9.4 T) spectrometer, equipped with a 5 mm probe, by using D₂0 forexternal lock of the magnetic field. Experimental settings were:spectral width 10000 Hz, pulse width 7 μs, acquisition time 10 ms, 1000scans and no sample spinning. The solution used contained 170 enrichedwater (2.6%, Yeda, Israel). The observed transverse relaxation rates (R⁰_(2obs)) were calculated from the linewidth of the resonance at halfheight.

(5) Single-Crystal X-ray Diffraction

Diffraction quality crystals ofN,N-Dimethyl-6-carboxamido-2,3-dimethyl-5,4-hydroxypyrimidinone weregrown by diffusion of ether into an ethanol solution at roomtemperature.

(6) Physical Measurements

The NMR spectra were recorded on Bruker AMX 300, AMX 400 or DRX 500spectrometers. Chemical shifts (δ) are reported in ppm referenced toresidual protio-solvent resonances. Melting points were obtained on aBuchi apparatus and are uncorrected. Electronic absorption spectra wererecorded on a HP 8450A or HP 8452A UV-Vis diode array spectrophotometerwith 1 cm quartz cells. Elemental analyses were performed by theAnalytical Services Laboratory, College of Chemistry, University ofCalifornia, Berkeley, Calif. Mass spectra (FAB+ and El) were obtained bythe Mass Spectrometry Laboratory at the College of Chemistry, Universityof California, Berkeley, Calif.

It is understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications or changesin light thereof will be suggested to persons skilled in the art and areto be included within the spirit and purview of this application andscope of the appended claims. All publications, patents, and patentapplications cited herein are hereby incorporated by reference in theirentirety for all purposes.

1. A composition comprising a targeting group attached to a complexbetween a gadolinium (III) ion and an organic ligand, said complexcomprising a structure according to Formula I:

wherein R¹, R², and R³ are members independently selected from a linkingmember, H, substituted or unsubstituted alkyl, substituted orunsubstituted aryl, substituted or unsubstituted heteroalkyl, hydroxy,carboxy, amide, ester, and amino groups, with the proviso that when A isnitrogen, R¹ is other than amino, and with the further proviso that whenE is nitrogen, R³ is not present; R⁴ is a member selected from a linkingmember, H, substituted or unsubstituted alkyl, substituted orunsubstituted heteroalkyl, substituted or unsubstituted aryl, hydroxy,carboxy, amide, and ester groups; and A, E and Z are membersindependently selected from carbon and nitrogen, said complex having asolubility in water of at least about 15 mM.
 2. The compositionaccording to claim 1, wherein at least one of R¹, R², R³ and R⁴comprises a linker attaching said complex to said targeting group. 3.The composition according to claim 2 wherein said linker has thestructure

wherein Z¹ is a member selected from H, CH₂, OR¹⁰, SR¹⁰, NHR¹⁰, OCOR¹¹,OC(O)NHR¹¹, NHC(O)OR¹⁰, OS(O)₂OR¹⁰, and C(O)R¹¹; R¹⁰ represents H,substituted or unsubstituted alkyl, or substituted or unsubstitutedheteroalkyl; R¹¹ is a member selected from H, OR¹², NR¹²NH₂, SH,C(O)R¹², NR¹²H, substituted or unsubstituted alkyl and substituted orunsubstituted heteroalkyl; R¹² is a member selected from H, andsubstituted or unsubstituted alkyl; X is a member selected from CH₂, O,S and NR¹³; R¹³ is a member selected from H, substituted orunsubstituted alkyl and substituted or unsubstituted heteroalkyl; and jand k are each independently selected from the integers 1 to
 20. 4. Thecomposition according to claim 1, wherein said targeting group is asmall molecule or a macromolecule selected from saccharides, lectins,receptors, receptor ligands, proteins, antibodies, poly(ethers),dendrimers and poly(amino acids).
 5. The composition according to claim1 wherein said complex is targeted to the blood pool.
 6. The compositionaccording to claim 5 wherein said complex binds to human serum albumin(HSA).
 7. The composition according to claim 1 wherein said complex istargeted to a liver or a component of the reticuloendothelial system. 8.The composition according to claim 1 wherein said complex is selectivefor tumors.
 9. The composition according to claim 8 wherein saidtargeting group is a sapphyrin.